Help! Swapped 302 heads on my 318.

If you changed heads there absolutely no guarantee the pushrods are the right length. None. Just because they all used the same pushrods from the factory doesn’t mean anything now you have an aftermarket cam and rebuilt heads. The spring height may have been changed during the rebuild. They may have machined the head surface to true it up, which would shorten the required push rod length.

The other thing is, 302’s are closed chamber heads. You have a cam in there, we don’t know what the piston height is, whether or not there are valve reliefs, etc. With the change to closed chamber heads you may be having piston to valve or even piston to head interference.

Some of that is kinda unlikely, but there’s a lot of stuff you need to rule out.