My truck/trailer combo is 26K lbs. It runs it around fairly easy(for 170 hp). It can run 65 empty/downhill, but 60-62 with trailer. I've been from Syracuse, to Indy, to BG, KY with it so I'm not afraid to take it far away. It gets around 8-9 mpg with the pedal to the metal all day with trailer.
The old 673 is pretty reliable, you can get a turbo version with 210 hp. There was also a 711 ci version later. Basically a bored out 673ci. It's the same basic motor up thru the 2valve 300 hp motor. You can shoehorn the newer stuff in there, but with intercoolers and such it gets tight. If you get the big hood model(like a B73 or such) then you have the larger radiator shell and more room for motor. There is quite a selection of versions to select from. My truck has unknown mileage, but I haven't had to add oil to it all between changes. I put a few thousand miles on it yearly.
As for tranny, they did offer a single stick 10 spd(had a splitter lever on handle instead of another stick). Called a Unishift. Most any others will be twin stick unless someone swapped in something else(like some have that can't drive a twin stick).
They had 5 spd(usually only in fire engine or gasser trucks), 10 spd, 15 and 20 spd twin stick trucks. Usually you won't find too many that have highway gearing out back. These trucks weren't meant to go that fast from factory(45 was speed limit back then). When I was looking for a truck that was one of the things I wanted. 65 mph, maxi brakes, front brakes. Alot of these trucks had only mech parking brake(which sucks) and no front brakes(which also sucks with no trailer and wet roads). Most anything you could put on, but I didn't have the room initially to do alot of heavy work. They had single over and double over tranny's also, so that plays into road speed. Mine is single over, and runs 65. There is website that tells all about mack tranny's back then. Have you seen my little driving video? It's posted here(plus a couple other sites).
I did the king pins(cheap and easy by the way), rebuild air comp, put 12v start on it, brake chambers,lines, etc. Lots of little maintenance stuff. You can get most anything you need still since the motor was used up thru the 80's.
Lots of information on web, it's really helped me, plus clubs to join. The internet is a good thing(most times). Heck, if I could, I'd sell you mine and get my long hood KW that I want> But zoning won't allow that sitt'n outside so I have to stay happy with my little B!
