Hi new here but in the past two weeks I have been dealing with the same issue. But I had to do it the other way around found a good tranny like yours swapped the guts into my old case for a 3.0L v6.
Turns out your case and all m5r1 cases inside are te same. Gear teeth count change in the gear cluster and they have their counter part input output shaft blah blah blah. Cases inside are the same; bottom Line. So you and your son have a nice weekend ahead. Since you have to dismantle 2 transmissions.
Remove the rmtm5r1F guts out, I mean all, because those came with an updated 17 teeth in the reverse gear cluster and the one your son picked up it’s prop 16 teeth.
You’re swapping everything from the rebuild tranny to your old case. There is a nice 1 hr plus long video in YouTube that shows you how to do this pro style.
The top cover that holds the shifter; that whole assembly is getting swapped as well and it just mounts up and bolts on your old case/bell housing. Your old/broken tranny the slider and fork are wider for the 1st 2nd Gear due to an updated 3 part syncro for 1st gear. This is the reason you can’t use your broken tranny top cover with its forks, one of them it’s different.
So gut your old tranny and place everything in order on a piece of card board or table. I did this and took pictures and Watched!!!! the orientation of your sliders and it’s carrier (ref. Front<- ->back) and kinda used it as a map to help me when I was gutting the good tranny parts.
The speed sensor issue
The newer version tranny you have came with an updated speed sensor and the output shaft cover got an update for that new sensor. You will have to reuse the metal sensor gear that comes in the output shaft of your old tranny and change it to the new output shaft from the rebuild( this one has a plastic one with threats) your broken tranny it’s a metal one with blocks instead of threats. You will need to reuse swap the metal one. You will understand when you open it. Again same housing a little updated but it’s made for the same parts. Watch out for a single ball bearing under these parts. Get a good retaining clip set of players. Ace hardware it’s got one for $15.
Also when removing the 5th reverse fork&sider do this as a whole assembly (remove the bolt on side of the case that retains that short shaft. Remove the fork and slider With that short shaft it rides on or you will be looking for a single ball bearing and very hard to find spring that it is inside that one fork; don’t ask how I know that, I don’t want to talk about it)No! I said.
On assembly of this part it will take you a few tries. You, like many that have done this will cycle that reverse slider without that reverse gear on and will take the slider too far off its carrier and the 3 little pins will drop out and out comes the whole thing to reassemble and drop back in. I did it 4 times lol. Trial and error.
At the end of the day it’s a manual transmission I don’t think the ratio change it’s going to change anything. you control that clutch and acceleration.
It’s a small work truck not a racing drifting ranger. Swap the guts the bell of the housing change but the part where all the guts go it’s the same. Sale the old 2.3 casing as aluminum, should get enough to get you the new output shaft seal at your local auto parts. You can’t use the output shaft cover from rebuild tranny; speed sensor housing has change. Use the one that came with your broken tranny and swap the one gear inside.
I highly recommend you watch that you tube video. It’s over an hour long I watch it 3 times before I did this swap on my tranny.
Good luck.
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