1991 378 looking for info

I start off by apologizing. Never posted here before and not too up on the forums. I'm mainly looking to get some info on what appears to be a bit of an odd-ball truck I have. 1991 Peterbilt 378
Full vin: 1XPFDB9X8MN306669
Engine serial number: 5KJ05173
I may be all over here with questions so bear with me. First things first, the truck is 8 lug. Anyone else have one that's set up that way? It's got 24.5 all around. I know it's not unheard of, just odd(to me). Was this something that would be spec'd that way for any reason?

Next thing is the air operated HVAC. I'm sure this has been discussed elsewhere but I was not able to find much on it. If I change the blend slider to anything but Bi-Level, I get an air leak under the dash. Is this a "seek and ye shall find" kinda thing or is it a common issue? The truck currently has no A/C as it appears someone was in the midst of converting from R12 to R134A. If anyone has a diagram for the air line routing for the HVAC or even for the dash in general, that would be greatly appreciated.

If anyone could give the basic specs on the engine as well that would be great. (I.e hp/mfg date/ etc...) I have zero mechanical history on the truck. What I was told is it has a fresh overhaul but nothing to back it. It's got a Holset turbo on it so obviously someone was playing in there. It runs and drives great and doesn't get bad mileage from what I can tell so far. 1.76 million on the clock.

That's about all I can think of so far.... any help on any of these questions would be awesome. Thanks in advance.

- Jared