I need some help I’ve got a 1984 ford f150 with the 300 6 cylinder I had some issues with battery dying I thought I might’ve just left the lights on but when I jumped it off it ran good drove it home bout 45 min commute then the lights start dimming so I’ve replaced alternator external voltage regulator redid all the ground wires coming from battery to starter including grounding to the frame but as soon as I jumped it off with cables attached it runs 14.3 volts but I take them off within 2 min voltage drops to 8 volts and truck dies checked all inline fuses and all of them test good with test light and ignition on help me out here I love this truck but it’s becoming too much

 

Sounds like a bad major ground wire connection to me.
I'd slow charge the battery overnight first then begin any further tests.