Since rereading what you said about the way it did when it quit, I'd bet money it's the timing chain. Making a noise like a train, clackity clack, was probably the chain slapping against the timing cover.
It may have done some valve/piston damage, but with the low compression engine, you have plenty of valve/piston clearance anyway. I doubt the valves would hit even if they were totally open and the piston came up.
If you feel you would rather keep this engine and fix it, pull it apart remove the broken chain. Put on some new gears and chain and give her a try. It would be a cheap fix, if this is all that's wrong or happened when it died.
But again, I can tell your heart is set on the HO. I am pretty good at reading between the lines. Your betweens are full of 454 HO, 454HO, 454HO.
Sure you could probably put a few bucks in it and be done, but what fun is that. You will always kick yourself and say, man, I could have had the HO in here by now.
Just messing with you. Do what you have to do. Sometimes it's better to follow your head instead of your heart. But, it's nowhere near as much fun.