A Grateful Dead skoolie fell into my lap, looking for information!

Hello everyone! I've been looking up as much information as I can, but I think I finally hit my google-fu limit. Here's the basics:

1981 GMC Bus Chassis

5.7L GMC/Chevy 350 (confirmed)

3 Speed Manual Transmission

VIN: 1GDH6P1A3BV568696

Body Number: 39815

Chassis Number: 75592

Some pics: https://imgur.com/a/L3ay8RQ

I've been trying to find more information on this thing and come up with very little info. I did find the body number, but the site to look that info up is behind a username/password system that I have no idea how to get in (https://www.thomasbusonline.com/). I was able to find one site there (https://www.thomasbusonline.com/ESP/Vinlookup.aspx) that gave me the body and chassis numbers.

At this point I know it's a carbureted 5.7L Chevy 350 and that's about it. I believe I need to find some place to decode the body number or order number to get the correct info. It has a floor shifter, but I haven't been able to look at it yet. I'd love to know what transmission is in it so I can bring the right fluid and volume.

Any info or links to sites I can research this would be really helpful. I'll be sharing more pics as I get them. I have some of the inside, however the previous owner's stuff is still inside so I need to clear that out before I share pics online.