Help! Swapped 302 heads on my 318.

I did not read quite all of this but here are are examples of causes of this problem:

First, you may not know the history of the engine block. It could have been surfaced at some point, and that makes the pushrods effectively longer

You likely don't know the history of these heads and THEY might have been surfaced. Same as above.

Last, when some folks do valve jobs, when you grind the head seats this "sinks" the valves further into the heads. Likewise, when you grind the valve, that removes material and causes the valves to be "too long." You are supposed to adjust for that, by taking some off the end of the valve stem.

So, with just simple changes, the pushrods "got too long."