Engine Compression Question

12:1

depending on the gaskets you use it will be around 12.1-12.2:1...because the 05 head has a slightly smaller combustion chamber

just thinking out loud here.
if the 05 combustion chamber is smaller with the 05 head wouldn't that mean there is less space or volume.
meaning the compression would go up instead of down?

yes.

a 12.5 piston with 06 crank and head is 12:1, due to the shorter rod and larger combustion chamber

put an 05 head on that and it goes up slightly to 12.1 or so due to the smaller chamber

so the 439 kind of "holds it own" against the 449 due to it has the .4 higher compression?

in a way, yeah. there is a different port layout in the 06 heads as well which help power delivery and throttle response.