Only and OMC Stringer will fit the hole you have, unless you want to do a lot of structural work. And the full mechanical Stringers were probably the best of 'em.
You can replace your 1982-1985 stringer 800 with another 1982-1985 stringer 800, and to get the right gear ratio, you need to get it from behind the same motor you have(3.8, 5.0, or 5.7)
Its a 4.3 with a 2-piece RMS crank, and perimeter bolt valve covers. If it ever needs to be rebuilt, you need to rebuild what you have. 1986 and later 4.3s have a different crank, and there are no couplers for your drive and a 2-piece RMS crank
to change gear ratios you would need a pile of shims and a bunch of custom tool and the training needed to set the gear box up. since most of the technicians that were trained on these drives almost 40 years ago have since retired, there are very few people that still know how to work on them, or have the tools.
look for a Stringer 800 upper assembly from behind a V6 so you dont have to buy a bunch of tools, shims, etc.
kenny nunez used to be an OMG Technician. will let him chime in.
If the gears are still good in your current upper gear housing and just have rust on them you could re bearing and get the gears bead blasted. I have done that in the past with good results. The most important part is to not lose any shims.
If you can get a genuine OMC service manual you might be able to salvage the gearcase.
You want 4-6 inches of rolling torque on the ball gear shaft and 15inches on the pinion housing with 006-008 gear back lash.
The service manual will give you plenty of help.
