1986 GMC Astro rebuild...questions

I recently purchased a 1986 GMC Astro cabover, daycab, twin-screw for hobby only, and discovering some things I'm questioning. Looking for some help and appreciate any answers, guidance...
6v92 Detroit, and I'm not exactly sure of the tranny yet...previous owner said "7 speed" and tag at back of it is hard to decipher for now. Shifter doesn't have a splitter on it--just straight stick. (Has been in farm service for some time I'm told too)
1. Should it have a HI/LO on stick? (haven't gotten deep enough into it yet to see if an air line is near to feed one)
Hendrickson walking beam suspension (has the H logo imprinted on the frame attachments and a number--net search has yet to find a match for me) and has ONE air bag only at the fronts of the first axle. More odd to me is on that axle are 24 inch wheels/tires, but the other rearmost axle and front have the 22.5s...2. I would think they all need to be the same?
3. Some residue can be seen on bottoms of both tanks (algae maybe?)
Very little fuel left in each tank so no worry there. Not running it until that's resolved...owner just installed new filters couple weeks or so back, and has only run it a few times to show potential buyers. Any "tricks" to clean these up here at home?
Thanks for any help.
Tim

 

Also looking for a few odd parts too... first, need driver's door opening latch...the one mounted at bottom of door...

 

Thanks for the input...I've learned a bit more...seems I somehow didn't look close enough at the suspension/drivetrain--I find that rear axle is just a "straight" axle or I'd guess it could be called a fixed "tag", not powered unit. Best I can read the specs posted in 4 sections on the driver's visor, there are two types of Hendrickson suspension listed, and all tires listed are 22.5. Still curious on why the 24.5s were put on the forward, powered axle. I can also see a difference in the sizes of the hubs, wondering if that means 22.5 wheels and tires may not be mountable?

 

Here is a little view of the back of tranny so far...

 

Here is the best I can get of the tag on transmission...