The 10.5 compression pistons and the closed chamber heads are a matched set that produces the 10.5/1 compression ratio in other words the piston and the combustion chamber produce the compression ratio. With a 360 head you have a couple things working against you. They are open chambered and not closed chambered. The 273 heads are listed as 57cc whereas the 340/360 heads are listed as a 70cc head. There's a lot of compression loss no matter what piston you use. Also because of the larger bore of the 360, the chambers of the 360 heads are huge compared to the small bore/ small chambered heads of the 273.
