Had the same problem and ended up damaging the splines on the new clutch disc. Filed down the ends on the disc and used wheel studs to slide the trans on. Went right on.

For clarity:
You chamfered (with a file) the transmission side of the leading edges of the broached spline hole in the friction plate ?
If that is correct, it seems a good idea. How could it do harm ?

The clutch disc already has a chamfer on the internal splines, he probably buggered it a bit and had to repair it.

Agreed, each spline of the friction plate is chamfered - but only in one plane(roughly like a chisel).
I think he filed a chamfer on the other two planes ?? So the leading edge of each spine is shaped like the point of a nail . That would be chamfered on all 3 planes.
This would require a thin file , lots of patience and a steady hand.
If all he did was repair damaged splines - good idea. But if he modified the profile of the leading edge of the friction plate splines, that is more 'interesting'.

Chamfering the splines on three sides would also weaken them. In reality, both the input shaft on the gear box and the disc splines are very mildly chamfered in a single plane, and if lubricated with the correct grease and properly aligned, will quickly and quite easily slide together. By far, the biggest issues we see when people have trouble getting the gear box back in is improper alignment and excessive force, which ends up in aggravation and damaged components. These things slide together easily with a reassuring "thunk" when it is done properly.

"These things slide together easily with a reassuring "thunk" when it is done properly."
That is good to know because it deters people from modifying the splines to prevent a mating problem if the issue is simply alignment.

Well, I've reinstalled just a few of them over the last 20 or so years, and I have never had to file or grind anything to do so. And while experience always helps, so does using long bolts with the heads cut off in two of the side bell housing bolt holes as guide "pins" to help get the alignment right the first time.

Sorry to bump an old thread but I was wrestling with my transmission last night and just wanted to add what worked for me should someone else's Google-fu lead them here like it did me.

Threading the two studs into the engine as mentioned here probably would have helped a lot. So does a buddy to have eyes on the front of the transmission while you shift it around in the back. Out of sheer stubbornness and/or not finding this thread until it was too late, I did neither.

I was able to get the transmission pretty close by eye but I was fighting the alignment of the transmission up and down (think a truck with a broken frame, the bed and cab are sagging in the middle). I had a jack on the engine and a transmission jack at the back of the trans that let me adjust this, but it was really difficult to eyeball and of course if it's not perfect it doesn't push in. What I did was take a random socket whose diameter was close to the gap between the engine and transmissions' mating surface. Using some of the longer mounting bolts I could get them started into the threads making sure things were pretty well aligned. The threads can NOT bind, you don't want these supporting the weight of the transmission yet or you can foul up the threads. Once in place I could jack things up and down and get a feel for the threads to tell if they were aligned with both the engine and trans. Once close here I took that socket (in my case, the round end of a 3/8" drive 6mm hex bit for the pressure plate bolts) and used it as a spacer all the way around. I inserted it in four spots (12, 3, 6, and 9 o'clock) and tightened screws and adjusted the jacks until the gap was even all around. Once it was even I knew things were aligned and I could start tightening bolts.

You have to be pretty careful doing this, the screws should be almost loose enough to tighten by hand the entire time so they aren't supporting the weight of the transmission. If the splines aren't aligned you could also damage them if you keep tightening and the trans isn't going. I took it VERY slowly to take the trans the last half inch or so to the mating surface on the engine, but once I started using the socket as a spacer to check alignment all around it was much easier.

Edit: to clarify, this isn't using the bolts to actually pull the transmission into alignment - I was using the 'feel' of the threads turning by hand to tell which side needed to move and then sliding things together by hand. The key for me getting the last half inch or so was using the spacer to check my alignment and things were easy from there.

I had use the tip from JFP and it works very well, just get two long bolts with their heads cut off and you should be able to align the tranny properly, you may have to rotate the input shaft a bit to align the input shaft with the clutch splines

Yeah, so this is more or less what I did but I was using the standard mounting bolts. Wish I'd read up on JFP's route because you have to get it pretty close to aligned before those bolts will have enough thread to go in, I probably would have saved myself a lot of hassle using some longer ones and removing them after. Using that socket as a spacer just helped get the side to side and top to bottom alignment just right for the last little push and then I felt it slide into place.