use the search function, there has been TONS of discussion on 727 vs. 904 trannys. apparently to fit a 727 you will have to hammer the pinchweld at the firewall down. I'm not sure about the crossmember.
Most folks will say that if you are running a mild smallblock, under 400hp, a properly built 904 will handle the power no problem, also the 904 has lighter internals (thus lighter rotating mass) than the 727. if you look around you might find a A999, which is a slightly beefier (internally) version of the 904. same case, just slightly better internals.
welcome to the site, this is an invaluable resource for A-body info!!
-tim