Engine Placement/Mounts???

[jackitup300]

I have a 70 chevy 1/2 ton 4x4 lwb body on a 72 4x4 3/4 ton lwb frame. the 70 had an inline 6 in it but I'm not sure what the 72 had in it. anyways, I have a small block in the truck right now with a th350 and a np205. It has tall valve covers and the outside edges rub on the firewall as is. I had a HEI distributor in it and that also rubbed on the firewall. The fan is a good 6 to 8 inches from the radiator. The transfer case cross member fit on the original holes and both stock drive shafts fit fine when I built the truck. My plan is to go to a solid roller 396 big block and I can not see how it would fit without modifying the firewall. I know these trucks came with big blocks but I don't understand whats wrong in mine truck for a big block to not fit. There are holes to move the engine cross member forward but they are only on the top of frame and even if I did that plus drill holes in the bottom, my front drive shaft would be too long and my rear would be too short. Looking for some input please. These are the pictures I can find right now to show my situation. thanks.