Head decision time

How far down in the hole are the pistons?
That in itself is a big factor in the head I would use.

The sp2p is a mileage master intake developed during the gas crunch of the '70's.
The stock intake is very good.

If the Pistons are not to far down, the 360 heads would be better. But do remember that a better flowing head is better than one with more compression. You will still make better power with better flowing heads than the closed chambered heads.

To get the 273 heads to flow as well as the 360 heads, you'll need to spend money on Geiger valves and porting. Cheaper to use the 360 heads.