That's a tight fit. I know, have a 73 Scamp with gen 3 and headers. Engine was already in when I installed the headers. It's possible to reverse the process, albeit a pain in the ***. With having to drain the cooling system anyway, your only a few connections and bolts away from dropping the whole damn thing out the bottom.
It’s possible. I just did mine a couple weeks ago. Driver side, had to replace lifters. Not hard, plenty of room, removing headers sucked. Installing sucked even more; but it can be done, get a flex ratcheting wrench, and you’re good to go.