Keep low diff 5.1 - 4.5 with th700 4 speed, this gives the top end in overdrive don't need the tall diff if you go to a 3.5 you are robbing your low down power that you need. Been there........... 
That's the way I'm planning to go. Mob in Melbourne do a TH700 4 speed auto that they reckon is strong enough for anything. Only problem is that it works out to be about 4k by the time I fit it into Beddy.
I'm toying with other sources including just getting one from the wreckers and bolting it in with the Dellow conversion plate and seeing how it goes. If it doesn't run right then I'll spend the money and get it reconditioned. I've lost nothing but time and a bit of auto oil to try it.
Neil
Don't forget to add the B&M Shift kit this will save the box and makes it a lot stronger so to speek, more postive change so you don't lose that much power and because the gears are not shifting over each other when they change the box stays a lot cooler, the kit comes with 2 set ups, For RV's - towing and Drag.
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