Wow! The longer I'm part of this forum, the more I like seeing the help that pours out when somebody needs it.
I'll offer a little on the 391/427 crank offered above. Yes it will physically fit in the 390 block as long a the snout is turned to match the seal diameter of the timing cover for car engines and not trucks. Also, must obviously fit a harmonic balancer for a car engine.
Aside from that the biggie is balance. As manufactured, it will not work with car rods and pistons. That said, if it has already been balanced for a 427, it won't take much balancing to make it work for a 390. Depends on rod and piston combination. The only other thing is oiling. From the factory the truck cranks are not cross drilled. No biggie here as 390's didn't have cross drilled cranks anyway.
In short, with very little expense, it will work.
Last note. Please get a steel
bellhousing. I don't care what anyone says about steel flywheels being bullet proof or "Oh gee why waste your money." You are building a performance car, designed with one purpose in mind......goin' fast! That means you are going to do things that stress parts....sometimes to the breaking point. Don't take a chance on your legs OR your life. $300 is damned cheap insurance my friend.
Have fun building your car!
.....and keep us posted on your progress.
Al