Engine swap?

Considering replacing the tired 225 in my wife's 63 valiant. It has a 904 push button trannny that was rebuilt by a previous owner. The current engine has a 2806830 casting # which tells me it's a 67-74. I would like to find someone's "take out" that is in good running condition. Are all 225's a direct fit? I need to keep this on the CHEEP because my wife says """Why would you replace the engine? It starts and runs fine!!!!"""". I do have to add oil constantly and it does run great. Yeah I've done all the tests, the engine needs rebuilt or replaced,,,,THNX!!! RH