Why did you use 305 heads on a 350? they have less flow. The cam is probably to much lift and the valves are hitting the piston. Whatever you do dont crank the motor over until you find the problem. Bent valves or a cracked piston.Excessive ware on cam also.
Do the pistons have a valve relief cut into them?
Did you do a clay/ measure test for valve to piston gap?
Luv the smell of NITRO in the morning.