I have a 5 stage dry sump system on my engine with the tank in the back (above the rear axle) I too wanted to move the engine back a considerable amount but ran into a problem, the transmission. Because I'm using a Zf6 with it's internal rails the shifter is in a fixed position and as it is now it's already pretty far back, just in front of the parking brake. Then again the engine is aready a considerable amount behind the front axle on the C3 corvette (about the only reasonably good thing about the chassis/suspension setup) and as low as it gets. I couldn't get it any lower without loosing a lot of ground clearance.
As for the oil pan, I have a stainless dry sump pan, it's custom made and has the 4 scavenge ports equally distributed, one in the front two the pass side front & rear and 1 on the drivers side rear so they are out there.
A big block is only about 3" longer than a small block, and that's using a long water pump so w/ a short one you win a little there.It's about 20,5" for the small block and 23,5" for the big block from water pump snout to bellhousing flange.