I thought I should explain why I said only minor grinding would be needed to fit the '77 sump motor in the '73 frame. That's all that was needed when I put one in a '68 XLCH frame. I forgot that the later frames have the cross brace about an inch closer to the front motor mount. I measured ROUGHLY from the lowest front mount bolt to the brace. 11-1/4 for the '68 and 10-1/4 on my '74.
When in the '68 there is 5/8 space between the sump and the brace.
The set up in the picture is without a rear motor mount. I believe that with one, the case would be a bit closer to the right frame rail.
Using my rough measurements it seems like it might be possible to leave the brace where it is in the '73 and grind it for clearance and reinforce it to the rear if needed. Maybe? Can't say for sure.
So apparently it fits without much trouble in a short frame.
Short frame = 1957 to 1966 all models. And 1967 to 1969 XLCH.