904 giving up the ghost?

I don't mean to be bumping this thread but I decided to wait on fixing my Duster's transmission until my truck is back on the road, or at least close to it (in the process of doing a 4-barrel conversion). I've been doing a ton of research in the meantime trying to get a better idea of what's going on and it's looking like I'll most likely need to drop the transmission; I still need to verify the hydraulic pressures but I tried driving it again the next day with everything cooled off and the symptoms seem to be pointing towards a pressure issue. If I put it in Reverse it seems to go into gear fairly normally although somewhat soft; any forward gears and nothing really happens, not the "thunk" like it's supposed to but if I give it some gas with the shifter in a forward gear it feels like it slips into gear and kind-of moves. I've also been thinking about how high the fluid level was on the dipstick; it was WAY above full, like 1" over and I know for sure it's not overfilled (topped it up last year to just below Full line with engine running in Neutral in my mostly-flat garage, also checked the fluid level when it started acting up with engine running in Neutral). That makes me think the fluid isn't being pumped properly and looking through my 1970 Plymouth FSM in order to fix that I need to get access to the front pump which would mean dropping the trans...??

I might do a hydraulic pressure test this weekend if I have some spare time but I don't want to start taking anything apart until I have all the necessary tools and a solid plan of what needs to be done. I want to minimize down-time for my Duster as much as possible because I'm trying to compete in time-attack events this summer and the first session is May 31st which I'll obviously have to miss but I really want to make the next one which I believe is mid-June. I also want to use this as an opportunity to wrap my head around how Torqueflites work in detail; reading text and looking at pictures only goes so far, I can't really understand how something works until I get my hands on it and take it apart myself.