Big Block/A-518

U do not have to cut the floor pan but put a good dimple in it with a rubber mallet to make room for presure switches. Just modify the crossmember up in the hump and weld a plate in the cab. And yes you will have to fab up a new lower support. Working on mine now, give me a day or two and I will post some pics. The support I made is a hybrid of Keislers 4 speed manual crossmember and what charger70 did.
Hot Rod magazine has done this...http://www.hotrod.com/howto/hrdp_0706_hemi_swap/
Also do a search, I have posted in a few threads and a few others have to about this swap

Is this a Lock up 518?
I got my torque converter from smr. www.transmissioncenter.net has what you need to make it shift automatically, not sure if they sell the conversion kit