I've been wondering about the aluminum main cap issue myself. In one of the other recent threads there was talk of bed plates, and at least one of those bed plates was made of aluminum, for an aluminum block production engine. I wouldn't think that a 200K mile intent engine would use an aluminum bed plate unless it was reliable over the long term. Are aluminum main caps somehow different than a bed plate application? I'm guessing the aluminum bed plate was used on an aluminum block to eliminate differences in temperature variation; a steel bed plate would grow less with temperature, and could put additional stress on the block.
All the aluminum FE engines I've used have steel main caps, which would be better in terms of "growing" with heat and maintaining main bearing clearance than an aluminum cap, but I'll bet you could run just a little tighter cold clearance with an aluminum cap and be fine at operating temperature. Sounds like this must be what the big block Mopar guys are doing...