I have never tackled butcher block tops, but my client wants one in walnut. The tops will not be subject to much abuse, but they will cut on them. I anticipate using salad bowl or mineral oil finish. One top is 100" long by 30" wide.
I visited the local hardwood lumber supplier and picked up 8/4 flat sawn, kiln dried stock. I will probably target 1.5" finished thickness. The client paid for lumber upfront, but I will not cut it without some words of wisdom.
Butcher blocks are a people pleaser - they are very showy and photograph well, and they are very time consuming. I assume you are doing an end grain block, so what follows is my method.
I like a brick-laid pattern, so my first step is to find a material width that yields the best use of stock, and looks good in whole or half width. This gives you a dimension for ripping and jointing your stock, which you then edge glue in manageable lengths to your rough width. These panels are thickness planed and/or sanded, then crosscut to the rough thickness of the block. If you have lumber with pronounced heartwood and sapwood, it's more interesting to alternate dark/light, which shows up dramatically in the finished surface.
Next, glue laminations of the strips in comfortable lifts - I like to do about six at a time to keep the glue from gumming up. It helps to pre-drill alignment holes for 1/4" dowels, which prevents the lams from slipping, keeping the pattern consistent. Next step is to plane and thickness sand to finished thickness and mount in or on the rest of your work.
Take pictures - butcher blocks can be very impressive samples. One of these days, when money (or my time) is no object, I want to do a herringbone pattern top.
I milled the stock a little oversize in "height" (read: width), by 1/4", after acclimating to the shop for a few days. It probably wouldn't hurt to mill in stages, let it rest a day or two, then do final dimensioning just to make sure it's stable. Joint one face and edge dead flat and plane the other face exactly parallel. The more exact you have your stock, the less trouble you'll have getting a flat surface.
On the larger, heavier top I did, I bored holes for 3/8" threaded rod about every 15 inches, and inserted as I was gluing up. No holes were bored on the two outside boards, so all the hardware is hidden. But tightening these bolts across the bulk of the width and throughout the length can help resist any natural tendency for joints to separate over time. I wouldn't crank them severely, but wrist tight seems to work. Use nylock nuts, too. You'll need to counterbore where the washer and nut sit. If you use edgegrain, not only will you have greater abrasion resistance, but your movement issues will be less with this configuration.
Clamping and gluing a top this large can be tricky to do by yourself, so I'd suggest getting some help. By all means, use a very flat surface to work on. Have a rubber mallet handy to persuade any boards that don't lie flat. My choice for adhesive on these tops is polyurethane glue. That makes help even more critical because you have the wetting requirement, then spreading the glue, mating the surfaces, and continuing that process long enough to get your 30"+ of width, while maintaining a reasonably flat configuration in the face of short open time. I found, on larger pieces, that clamping as you go helps. Of course, in doing that, someone needs to constantly take off and apply clamps while the new boards are being laid on. That may not work well on a top quite this long. But for my money, polyurethane will withstand prolonged moisture better than anything.
Gary Rogowski has an article in Fine Woodworking (Dec. 2003, vol 166) on gluing up tabletops. It has some clamping advice I've used and found helpful. Once everything is assembled and clamped, the worst will be over. You can trim the ends with a circular saw and clean them with a sander. Trimming the width on something this large when completed will be difficult, so you ought to adjust the "width" (read: thickness) of several boards as needed during milling to arrive at your final desired width. You could use a circular saw here also, but I'd try to avoid that. Flattening the top would be best done on a widebelt sander. If you don't have access to one, you can use winding sticks, rent a portable planer and use a belt sander, and get a reasonably flat surface the old fashioned way.
Your finish ideas sound fine, but I will point out that most finishes will be non-toxic once they have completely cured and the volatiles have completely evaporated. I've used Danish oil on cutting boards for years, and never had a taste or illness problem. Having said that, I've heard that Tried and True oil is also highly recommended. I think it's a non-additive linseed oil that's supposedly great for such applications. Chris Becksvoort uses it and has/had a video clip on Fine Woodworking's website explaining its merits and his application technique. Might be instructive to check that out.
One thing I didn't account for in my overall job was that a board which is stable in a 6 inch width may not be stable when ripped to 1.5 inch widths. I watched strips bend right as they passed the table saw blade. End result: I was really scrounging for wood for the last glue-up. Live and learn.
I had to let the wood sit for a week after rough sawing before final milling. With anigre cabinets, it looks like a million bucks. I figured that I would charge someone $45 CDN per sq. ft. for the final product if I were to do it again.
Comment from contributor J:
In regards to the risks of the toxicity of walnut; the toxin Juglone, which gives walnut its Latin name, Juglans Negra, is mostly toxic to other small plants. A tell tale sign of a walnut tree in the woods is a circle of earth around the base where the root system has contaminated the soil - preventing any other plant life from growing. This toxin is nominally harmful to humans in small doses.
My shop regularly does butcher blocks in Jatoba, which we all know is fairly toxic. The toxicity issue for both Jatoba and walnut is the dust generated during processing the materials. Once the butcher block has been completed and finished with a good mineral oil, the risk of toxic contamination to food and humans is nominal.