unless it is an a/trans frame to begin with you will have to remove the crossmember under where the bellhousing was to accommodate th400 and install a rear trans crossmember. they did make an orig rear trans that was pretty stout(Picture below)
those frames are pretty good. all frames will flex. it all depends how one feels about it.There are people that will wear both a belt and suspenders at the same time.
how are you going to transmit that power to the ground. are you going to use 20" wide tires and what type of suspension. depending on those you may have to make alterations to the back frame. people can't guess at what you are going do in order to answer you properly.
this is out of a 66 and note that it bolts to the top and the bottom of the frame
ron