To use those Sportsman heads with their 64cc chamber on the 400, you need to find a piston with a 17 to 21cc dish volume to stay under 9.75:1 compression.
On the 327, a 5cc eyebrow flat top piston will get you right around 9.5:1 depending on head gasket thickness.
The 327 won't be any where near as much power or fun as the 400 IMO. Easily the 400 will make 100 more HP and torque, and be a lot more low rpm street friendly doing it.
When you look at pistons, watch out for reduced height "rebuilder" grade pistons, they will murder compression ratio and kill good quench characteristics at the same time, making the engine MORE octane sensitive even though they deliver a reduced compression ratio..
Stock 400 Compression Height is 1.560" with a 5.565" rod, if you use a 5.7" long common SBC rod from a 327 or 350, then the correct CH spec is 1.425".
Correct CH spec for the 327 is 1.675"