[tornadoalleycruiser]

That looks like the longer tail shaft H42 with the spacer?
I"m not sure if it's splined all the way up to the back of the tranny like the shorter H42 or not. But.. Here's a little tidbit I ran into on my H41 toybox. I swapped over the FJ60 top cover for an FJ40 one and the original reverse light switch was too deep and hit the adapter plate. Check that.. I think I had to use the FJ60 reverse light switch which is on all the time until you hit reverse.. Still haven't fixed that on mine..

If it's splined all the way thru then remove the spacer behind the tranny, check your clearance on the splines of the output shaft that go into Marlin's 19spline input, Put a clamp collar on the tailshaft of the tranny to keep the output shaft from walking back in and your done. For safety sake I had a 10spline clamp that had been machined by marlin which was different but I used the thickness.. Ordered the correct metric clamp collar from McMasterCarr and chucked it up in a lathe and matched the thickness of Marlins.. Don't think I needed to do that but didn't want it to interfere.
Great progress!