emi big thunder are great have had them on a 454, 489, 502,540, 548, making as much as 620hp

no one has mentioned that a 4.250 crank is a tight fit in a standard deck.use a tall deck block if you want it to last.

The engine wasn't as bad as I thought. Think I'm going to use the stock crank and connecting rods since they are forged. Can just hone the cyl. Put a higher dome piston in to bring up the compression. Port and polish the heads and the intake manifold. get a valve job done and put a bigger cam in it. I was thinking of harden marine low exit system. My thru hull is under the swim platform so I need a little bit of a different set up.

Make sure to have the heads pressure checked and magna fluxed!!!
I had a 454 mag that only had 420 original hrs on it that was in fresh water only until the last two seasons of it's life and then it was brackish water but cleaned and flushed. #4 cooling passage ate through around the intake side and water locked the engine. What ever happened on your cylinder make sure you resolve what caused it or you will be right back to square one fast.

The original forged stuff is good. If you find someone, maybe even here, that has some OE forged pistons you could replace the one and go if it is not too bad. If you have to get into a set I think you will find that most newer pistons are a slightly higher height number which will bump the compression ratio up just a tad but I would not target any sizeable increase with those cast iron heads or you will not be able to run on pump gas. More cam will do a lot for you but make sure you don't exceed your exhaust or you will get reversion and hydro the engine. A roller cam will do wonders too but you will have to get the bridged followers for that early block to run rollers. Get a good marine cam that can work with your exhaust and will run the fuel grade you want and that is the power level you will have. You will need to change more stuff if you want to get into high HP but this mag can give some good power with a just a little work.

Keep on the lookout for Gil manifolds, last year I picked up two sets of takeoffs for $800. Huge improvement over those stockers and I sold the extra pair for more than I paid for both on eBay!

Lots of Lightnings for sale in the Swap Shop too over the past few months. Someone had a set of very nice looking all stainless Lightnings that had the low transom exit that I think you are looking for. Another benefit of this is that the steep drop is an extra hedge against reversion.

Two sets for $800 is a great buy if they aren't leakers. I would buy those just to flip them if I came across a deal like that. Usually you are doing well to buy a single pair for that price.

If anyone has a set of low hour standard bore 454 Mag pistons I would be interested. Would buy the rods too. I have an excellent 454 Gen VI block that is standard bore with zero wear in the cylinders - would like to stay standard with it and build it as a spare motor. PM me if you have anything.

Yeah, I got lucky. The guy swapped them out for lightnings, any they were low hours, no leaks, with tails!