There's a build thread out there somewhere on this but I had the rear of the engine only about 3/4 of an inch in front of the firewall and I believe I only hit the swaybar which was relieved by pounding the pan in only slightly.
I used a first generation maxima oil pan which is a front sump pan but I think the datsun 810 had the same pan.
You might be able to do some experimenting with crossmembers for a rear sump engine like a Z31 but not sure about the rack's distance from the control arms with that setup.
Here's a thread from hybrid done pretty recently.
l28et swap in a s13 240sx - HybridZ