Help! Swapped 302 heads on my 318.

Aftermarket crane cam and stock 318 springs.
Not likely a good match.
A stock 318 cam, which is what the stock springs are designed for, is under .400 lift

Also, depending on the lift, there’s also a possibility of retainer to seal interference.

Fundamentally....... it pretty much has to be one of these things.......
-pushrods too long
-springs coil binding
-retainer to seal interference

Since it fired, and clattered, and shut off...... and now has no compression...... based solely on that, my guess is pushrods too long.
Valves hit pistons after start up(the clatter), now the valves are bent(no compression).

I’d turn the motor over until both lifters on number one cyl were as low as they can go(tdc , #1 firing).
Then slowly remove the rocker shaft bolts and see if the valves move(close).
That will at least tell you if the valves are being held open.

If they don’t move, it’s time for a leak down test to verify the valves aren’t bent.

Even if they do move...... well, still best to do a leak down test.

You should also measure for the correct pushrod length and see how much different that is than the pushrods you’re using.