Kelly,
I wouldn't hesitate to use an early 20b. The early 20b housings are plenty robust enough for a/c use. The beefier irons could be an issue if you're building a turbo race engine due to the higher loads imposed during up shifting and down shifting. We don't experience any of those loads.
I'm pretty certain that my 20b is an early model and I have not experienced any issues with irons cracking around the dowels. The weakest area is along the inner walls of the o-ring grooves nearest the water jackets. If your o-rings aren't to factory specs, you run the risk of cracking an o-ring wall which ruins the housing. Been there, done that. If you treat the engine with the same care you would a certified a/c engine, then you won't have any trouble with longevity.