Mild 360 Magnum Build

not stock knock offs
re-builder pistons with reduced compression height (compensated pistons)
you want to make hp on regular gas you need some tight quench
open chamber heads take rpm (more cam, gears, converter) and high octane to make horsepower
I helped design the quench head pistons/ step dish pistons working with John Erb at KB- others smarter than I - some with propane experience- a real genius, and Speed-O Motive (they have left the strip I'm sad to say) also helped to solve the problem with the MOPAR open chamber heads. Speed-O-Motive used to weld up the chambers- it's that bad
what cc your heads- have you checked them yourself?