I am just recalling the bike I made for my dad a while back. An older Schwinn mountain bike, 21" frame. It had zero flex issues and my dad is 6'4 and was 235. In earlier times he would snap pedals off spindles. There are several large cromo frames out there, not the least of which is the Zion EBB. The dropouts are vertical notch(like standard mtb) and the tension is made by an eccentric BB shell that a standard crank unit can mount into. Thus it eliminates any of the axle sliding issues.
As far as cranks go, hve you looked into the newer Shimano units such as the Hollowtech II 2-peice design?
Considering all the stuff I just listed is freeride dirt jump rated, a normal person even of great force should be able to work it.