I've always used Dextron (as Tucker recommends) in my hydraulic system on my '77 1742 and it's worked well. About the closest thing to a problem I've had is that if it's very cold (say less than zero F) and I don't let the cat run for 10 or 15 minutes before I move it, the Dextron will foam a bit and often overflow the hydraulic tank
And thanks for the pics of the plow mount on yours. I was the one who asked for the photos. Interesting, but I don't think I'll be building one for mine. I found it interesting that there didn't appear to be a way to remove the mount (other than cutting it off) and even with the blade off it appeared that it might get in the way in rugged terrain.
Your 'cat is a nice lookin' one - especially for one that's 30 years old. It's not running the original grousers - the four-bolts-welded-in style are fairly late model.
Based on the model number on the spec plate (1642) I'd suggest that your 'cat started life as a pickup cab.
All in all, its a really nice lookin' unit! I'd be interested in seeing photos of the interior of the cab.