I put a W50 in mine, they arent as strong as Dellow reckons because a stuffed 5th inmy first one. The owners manual for the Mustang back Celica actually says not to tow in 5th and go easy on 5th up hill. How rare is the W55 now?

4.56 vs 4.10 Your B20 has a limit on how fast it can accelerate, hold it flat in neutral and time how fast it takes to go from 2000 to 6000. You find the stock 3.13 x 4.10 is marginally slower than 3.56 x 4.56 and you will find taking off that the 3.56 x 4.56 will just blow the tyres off when you start on dirt and a short shift to 2nd is necessary. 3.13 x 4.10 is slightly hard on the clutch, 3.56 or 3.28 would be better. When I put my 4.56 in, I had switched to an earlier 3.28 1st and I would short shift to 2nd which was a slight improvement, then 3rd and 4th are actually fun with a 4.56. My friend ran with 3.13 m40 x 4.10, and was best at the place we ran at, but the 4.56 made 3rd, 4th and 5th a lot of fun.

With the box you have, a 4.56 would be annoying, W57 3.28 would have beem maybe Ok, but you still dont have a proper OD. W58 would make a 4.56 good, but they are very rare.

I have CIG Locker in my 4.56, but the hand brake parts are gone. With a locked diff, half a turn of steering lock stop it rolling down a hill.
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Three 164's, Two 144's, One 142 & a partridge in a pear tree.