First, let me say welcome to the forum! Your project sounds pretty sweet.

That said, the answer to your question is probably not what you want to hear. There is no magic answer or one-size-fits-all measurement. Everyone's install is unique, and as such, you'll have to determine your own best placement with regards to floor and firewall clearance, radiator clearance, header clearance, steering linkage, crankshaft/pinion angle, oil pan and crossmember clearance, etc., etc., you get the idea. You really need to have all these other questions answered first, then build your engine mounts last and placed in a position that allows all the other items mentioned to work together.

Good luck with your project, and remember we are always here to listen and help.