Tyler,
I think the voice that is screaming the loudest is saying "don't do the widen-the-body thing to get 345s under back of the car!!!" Hell, I can hear that voice screaming all the way out here in Maryland. If your car weighs less than 3000 lbs., then 315s are gonna be plenty of rear tire for road racing. Find a way to that damned transaxle under the fenders on an 11 rim and finish the car.
Engines come and go. Get the car on the road with a cool, reasonable, and solid setup under the hood, and feel free to change it later. See "The Mule" for any further information on switching engines after debut.
I think you're just looking for confirmation for what you know is right. And buddy, you're right.
jp