There isn't any concrete numbers. If you have an HEI dizzy, move the engine back enough that you can still move it, maybe 1/2 to 3/4" from firewall. If you have the small cap dizzy, you have more room to go back. If you have a stick tranny, make sure the shifter comes out of the stock hole with enough room to go through all the gears. If you have an auto tranny, it can go anywhere in the tunnel. As for the tail end of the tranny, just make sure it's centered in the tunnel.