Does nobody make custom gears for the 8 3/4? I'm sure the OP would rather pay to have custom gears made for his rear than go through the effort, time and expense of swapping a different rear.

Gear vendors on the tailshaft.

2.50 is a fairly common gear for 9" ford in the junkyards. Just another option if you are ok with ford junk in your car.

A custom gearset is going to cost you as much as a complete Ford 9" setup. I personally would just run an overdrive. If you truly have gone through the trouble to to fit a 4BT in there (suspension, cooling, fuel system, and I want to know how it will fit with a hood on it and a stock K frame), the work and expense of an overdrive swap is cake.

I also want pix.

Holy Sh*^@!!.
I haven`t checked in for a couple days!
Thanks for all the suggestions. I really want to keep the 8 3/4 there, it is super clean, dry, and still has the factory paint splotches on it.
I guess the 8 1/4 is my cheapest option so far if 8 3/4 gears can`t be found. Gear Vendors OD would be perfect, except for what it costs. I can just slow my cruise speed down to stay in the RPM sweet spot and get by fine.
As far as the work and time I have in the swap, I could do it again in 1 week easy (40 to 50 hrs). the pay back is huge in fuel savings. And the used 4BT (70,000 mi.) costs around what a good smallblock rebuild does. And it will go 7000 hrs with good care.

The Cummins fits real nice once the oil pan is modified, and the tube from turbo to intake is re-routed to keep it low to fit under stock hood.
Cooling not an issue with 4BT, using stock radiator.
As far as suspension, engine w/accesories weighs 60lbs more than B or RB w/acc.

K-Frame, start by removing anything that looks like an engine mount, and fab new ones. Not difficult. Otherwise K-Frame&steering is factory.
Sorry there`s no pics yet, but look for them later,I promise.
Or see the car in Spring if you`re I Farmington MN at swap meet.

I forgot about fuel system, original tank & 5/16 line, only need to run a return back to tank.

What kind of fuel mileage does it get on the highway like it is now? What kind of vehicles do you find 4BT's in? This is very interesting

As it sits now, and with cold temps, approx 22mpg. @60mph.
This engine was in a Potato Chip Step van. Even a lot of the 70`s step vans that were built on GM chassis were converted to the 4BT (originally 6cyl. gas). The same engine is in 1845C Case skid loaders & some Case backhoes. I have seen telehandlers with a 4B in them.
I have a complete `77 GM chassis step van with a 4BT in it that was built with a straight six gas. eng. and converted about ten years later.

Currie has used 2.50 gear sets for 9" Fords for $99.95.

Stock style 9" housings go dirt cheap.

Now why can`t I find those gears for a 8 3/4?
2.4 would be even better.
I would even go $100.

Common ratios for 8 3/4s are closer to double that price. Plus, they aren't used in motorsports like the 9".

My reply was in regards to the 9" gears for $99.95
I was trying to make a joke about the $100.00.
Sorry.

Yea $95 1987, At the public works town garage they had fleet rear 8 3/4" and 9" fords lying out drum to drum . Rebuilt units and all different lentghs.

350" gm's
360" cryslers' a plenty
833's
granny gm's

knock the rusted piston free with the head off, air hammer together with a cheap cam. Float valves at 4500rpm. fast for $250. Been there, may still BE there ?

See if you can find the auto tranny out of a 5.9 truck. It will give you the OD you want and not kill in-town drivability.
The tranny will bolt up to the SAE bellhousing adaptors used on the B5.9 engines. I'm sure it's the same pattern as the 4BT.

Aren`t there some clearance issues with that OD trans in the car trans. tunnel? I don`t want to cut my floor.
But yes, it will bolt up, if it will fit a 5.9, it will fit 3.9. That goes for accesories on the front case too. Compressor,vacuum pump, power steering, etc.

floor clearance with the 518 will be determined by the engine setback. If the back of your engine is about where the original was then you could have some issues.
I have seen people solve that problem by trimming off the wide ribs on the tailshaft housing. I have not heard of the tail shaft housing failing after being trimmed. The 727 had a long tail without the extra beef on the case.

Going with a crazy high gear set will give you really poor in-town performance and mileage will drop considerably. An overdrive of some sort is the best solution.

My bell housing is in the factory location, I`m using stock trans crossmember. But was the 518 used with a 5.9 Cummins? I can only use what will bolt to the plate behind my engine.

As far as driving around town, the Cummins doesn`t mind lugging the wagon around at very low Rpm`s, it`s not like a gas engine. Different kind of torque.
With 31 1/2in tires & 2.76 rear, takeoff is OK. With around 2.3 gear and 28" tire, it will be similar. I`m past the experimental stage, just need to get it done. Thinking maybe an 8 1/4 swap with 2.2 to 2.5s