can a 70cm rim be built strong enough for a 380# guy?
or should i just stick with a 26"
As long as they're of decent quality, they should be fine. My 32h 700c wheels have been fine so far (I was 385 when I started riding 8 weeks ago) - the front one needed truing after 50 miles (LBS reckoned it was never properly done the first time - long story involving another supplier that my workplace has a deal with) and the rear is holding up fine.
Longer term I'll probably get a set of 36h wheels made up, but it seems that there's no immediate hurry for that...