late model 318 904 and 63 /6 904 question

The 904 out of my 72 had the 5 disk front drum. Not sure it came that way originally or was beefed up somewhere down the line. BUT the front drum on your 63 transmission almost certainly is only a 3 disk, so the potential for a upgrade is there. Also, if you swap the front pump, input shaft etc... into your 63 case you'll be able to use the more common 68-up converters.