Is this frame OK for expedition?
MTB frame built with DB, oversize Reynolds 631 tubing. My main concern is the butting of the top tube and DT: 8/5/8. Would it stand the rigors of expedition duty?
Assuming that the rest of the bike is suitable for a long trip, yes. Many old school MTB's make excellent long-haulers without much re-fitting. They were known as having lots of tour bike features such as long wheelbase, long chainstays and tons of hardpoints for cages, racks, etc. Some of them may ride stiffer due to having a 1-1/8" steerer. Those with 1" steerers rode pretty good because they had thinner fork tines with more forward curve - like a tour bike. Huh...
I ride mine on very long day trips and centuries. No tours yet. It's a 1992 Trek 930 hand-made in the U.S. of lugged OX2 True-Temper tubes made in America. That is the equal to Reynolds 853. It has skinny slicks on it and a sprung Brooks. I replaced everything with all the guts off my dead '88 Trek 830, because it's smoother and quieter than the newer stuff (as good as my LX and 105 on my Trek 520). I say it is the equal of many more expensive bikes. At the heart is a Sakae 48/38/28 Ovaltech crankset that allows me to haul a$$ over hills with a smile on my face. Shimano BioPace is the same thing.
Go for it.