I did this same upgrade but thanks to a previous owner mine may have been a mis step. The stock blower motors are right small comparitively to the upgrade;
In my case since 'Black Betty' is 30 + years old as is yours, some previous owner had done this prior, although I was either dealing with a worn out moter or the wrong one as they appeared the same but output was vastly different;
The correct part number is PM102 (Siemens) for a '73 Chevy Blazer 4x4 with a 350 SBC
without air conditioning. I struggled to find one not made in China, but ultimately I found one made in Canada that I had to order from NAPA.
If you have the original smaller motor you'll have to remove the heater box which entailed for me removal of the batteries, & battery tray to gain easier access to the motor and one of the nuts hidden under the battery tray that holds the heater box on. One of the other folks whom have done this reccomended to build a spinning jig to hog out the hole for the larger motor,
...but I have a Dremel tool with a carbide cutting bit so the little clearance I needed was easily trimmed away.
Since I had to pull the heater box out, this was a perfect time to clean it up and repair the old dried out flapper door seals and I did a replacement brass and copper heater core, as it was such a hassle to pull the heater box. When pulling that heater box don't forget to first disconnect all of the cables and wiring, one cable you may miss will be hiding between the passenger side kick panel and the heater box itself. Take your time with this and if the heater box gets stuck, just walk away and sip a beer. If you try to force that box out the old brittle plasitc it's made of will crack and break. Certainly something you don't want to happen as a completely sealed heater box is your goal with these.
Also one of the available motors PM105 (Siemens) with a/c seem to have a longer shaft than you'll have to grind down, to the comparative length of the original shaft, so avoid it if you can, it'll save weat and tear on your file
Here's what the box on mine looked like after I pulled it from 'BB'
after pulling the motor out.
....and as you can see, the seals were pretty dried out and funky. I redid them with weatherstripping I got from the hardware store.
'Black Betty' isn't my primary vehicle so I took my time with this upgrade taking four days to do it, once I got the right fan motor from NAPA (took two tries). I think one could easily do this over a weekend if there's no distractions. The primary difference since I did this upgradeis that the defrosters finally work!
I may be missing a couple of things here as I did this upgrade a few years ago, but it is a common upgrade and I'm sure others in this forum will chime in as this is the very best forum for Jeeps on the web in my opinion. Above all, take your time and be patient especially when pulling that heater box.
what else/mod did you have to do to fit it.
If you live in cold country this is a must. 
