If you are determined to make your engine 500 hp with minimal cost you may want to consider just throwing on a procharger kit. If you go the blower route, you can, but you need to buy more parts to get the same effect. Either way you are looking around 4-5k or so if you do it correctly. I still say you are better off buying a used motor that has all the right parts in it already. You are going to spend damn near that anyway, in the end

forget the blower.. if its running now and not blown up, you can make 525-550 on pump gas, for less than the price of a blower [ diferrent story if components were busted internally], and the headaches of just bolting a blower on.. trust me.. BEEN THERE DONE THAT.. Send it out to a reputable shop.

it a 1979 motor with 438hr no smoke
no knock no leaks been babied but prices im getting to get 500 to 525 are 5000 plus . blower 1800 to 2000

If your engine is really making 420 hp you should be seeing 72-74 now. I put a 525 XR/Imco in mine. It runs low 80s anytime. 85 with perfect conditions.

Low hours for sure, but if you put a charger or blower on that setup at that age you will probably blow seals and or gaskets. They have to be pretty brittle at this point. Keep that in mind. At the least you just might want to pull the engine and put some fresh rings/ bearings/ seals& gaskets/ and maybe springs also have the valves looked at. The cost in doing that is minimal.