I would be looking at upgrading the diff to something stronger.
Put some big tyres on the back of it for now, that will get your speed up and revs down, keep in mind it will have your speedo out by heaps as well.
Isuzu impulse, they have 3.9 rear gears and if you get your diff centre out of a manual one it should be LSD. The diff centres are the same in Isuzus thru till 87.
How long it would last behind a 350 im not too sure.
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Current project, 1979 KB Isuzu Ute.
Daily driver, '86 Rodeo Spacecab.
Chasing parts for Isuzu Impulse.