I have a 76 620 and the problem im having is with it running correctly

This is what happened been driving every other day with no problems parked in the garage
then let it sit over the weekend
mon went to start her up and dead as a doornail
go to jump truck off and it fires right up but when i take the cable off the battery the motor starts spitting and sputtering
giving it gas effect the rpms none it will just barely run .......

I know the battery is dead but the truck should run off the alternator just fine but it doesnt so im thinking the alternator is dead
but i dont know it might be the voltage regulator .....

So I guess out goes the old and in goes the new
and by new Im thinking of modern with built in voltage regulator
any help on doing this or advice on what to use or what would fit on the motor

thanks

sounds to me like you got it decently figured out... not sure on an exact part number for ya though... welcome!