Were do I start!
Fuel boost diaphragm on injection pump failed so lack of engine power. Converters need power and speed to lock up.
Do a stall test engine flat out, hold brakes hard stick it in 3rd or 4th and engage forward drive, with the boom in the air and transmission guard off look out the side window and watch the engine to trans propshaft it should not be turning at all during a stall test. if it is then clutch packs and seals want doing and most definalty the converter pump too.
what does the transmission pressure gauge on the dash say? 150psi is good less than 100 is very bad. (they are not all accurate so best to plug another gauge in to compare)
could be worn torque converter, worn converter pump, worn clutch packs and seals, leaking worm gasket under forward/reverse solenoids. or a combination of all
proberly time for a complete trans over haul. so engine out to get at converter and transmission out to do the clutch packs not a five minute job.
the cheap skate way out be recon the transmission unit and fit a new converter pump with out taking the engine out but best to do the converter too.
take the transmission filter out is the oil in the bottom of the canister grey and merky? Metal if so. there is also a suction filter in the bottom of the transmission behind the big suction hose at the bottom of trans.