I'd say your problem would be associated with the fact that you have put an 8 cylinder engine into a vehicle that has a tacho setup for 6 cylinder engine. This is assuming you use the normal Cruiser tacho. Doing the maths your 3500rpm at 100kms would equate to an actual value of 2625rpm at 100kms which is what my HJ75 Utes sits on(same standard 4.11 diffs). I'm assuming this has a 5 speed which came out sometime in 83 in the wagons.