My $0.02. The manual trans is not that difficult to install. You will only need to run another master and slave cylinder to actuate the clutch. That said I have an automatic in mine
I did so because at my age I prefer to just drive. Performancewise a manual valve body and higher stall converter will accomplish alot to get you close to the drivability of the manual. Confused yet?
Enginewise the best handling cars I have seen and driven were smallblocks. You dont need a tremendous amount of horsepower to make a rocket out of one of these cars. Quite frankly performance is all about a balanced car not one that has so much power you cant brake, accelerate or corner. Smallblocks will also give you a considerable weight advantage which will slightly impact the effort needed to turn the car.
I strongly suggest you do unassisted steering. While they are sometimes a bit heavy to steer around parking lots, once you get the wheels turning the steering is just fine...
Good luck with your build