Has anyone actually checked the difference between a WRX box and a GX/RX box? because apart from 1st gear a 1981 dual range is virtually identical to a 2007 WRX in gear size, I really doubt there would be much difference strength wise apart from possibly the clutch.

When we put the dual range box in the WRX offroader I had - provided I was sympathetic with 1st (which because of the dual range set up is much thinner) it would take whatever I threw at it from 2nd onward in high or low, we tried putting a hydraulic pull clutch set up in it above the starter motor because it had an 09 flywheel and h/d clutch but couldn't get the throw right so ditched that idea and went to the original cable set up which lasted as long as I had it......but I did always carry a spare cable just in-case

Surprising how many parts are interchangeable with pretty much all 5 speeds right from the earliest to the latest.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.