9.5” or 10.5” Bell housing....opinions?

Go with the 10.5 Keith. You will be ahead of the game when that 318/360 gets upgraded to a ????. lol.
They are pricey but go diaphragm clutch if ya can. They grab instantly, last forever if not hitting strip constantly and you eliminate large clutch pedal spring.

I have a Centerforce II clutch behind my 340 in my 66 Valiant and I love it...shifts like a new car, nice pedal and way smoother than my dad’s stock 66 Barracuda 4-speed.