It's hard to get worse than 20 with a 4BT. The bread trucks that most of these engines came in weighed several tons, were as aerodynamic as a barn and ran 3 speed autos or 4 speeds with 4.56 rear gears. The bread truck engines spend most of thier lives on the governor and they still got 15 MPG or better.
nevrenufhp is right about other adapters out there. There's ways to bolt a cummins to just about any transmission. The Cummins makes quick work of most autos though, I'd strongly recommend a manual. If you're going to bump one up, you'll also want to go with the dodge NV4500 bellhousing and tranny. With that you can run a 13" clutch. The SBF style adapter limits us to a 12" clutch that, even with the best organic clutch, will slip over the 300 horse range unloaded, less than that towing.
300 horse and 700+ lb/ft is very easy for under, possibly far under $500 in ebay/craigslist parts. What the guys bumping up thier 5.9's don't want is what the 3.9 loves.
As far as starter and gauges, if you get one from a a ford chassis bread truck the sensors are correct for our trucks gauges. The starter has it's own high capacity relay built in, the fenderwell mounted one won't handle the juice. I just run a 10 awg wire from the existing starter relay to the "S" terminal on the starter. I make all my battery cables from 4/0 and 6/0 welding cable and solder and shrink tube the ends. The 4BT takes a lot of juice to start, but they start instantly. One big battery will start it fine. Just a key tap and VROOM, RATTLE, RATTLE.
Another thing worth mentioning, These engines are very heavy and very efficient. They hardly need a radiator. The stocker 3 core 300 six radiator with a small electric fan is all that's needed to cool one of these.
Probably the most expensive part for the conversion is a power steering pump. Most of the breadtrucks only came with a vacuum pump for power brakes. Most of use like our power steering though. I try to run the first generation dodge 5.9 saginaw pump, I've been lucky and found them with the extra return for hydroboost. These pumps can be upwards of $150 on ebay and sometimes very hard to find. You can usually find several combination PS and vacuum pumps for sale on ebay, but these are almost always $200++ and they are very expensive to rebuild when they go out. I figure the point of the diesel is simplicity so I stick with the standard, proven saginaw pump and convert to hydroboost power brakes.
1970 F-250 4x4 original Willock swivel frame chassis '93 5.9 Cummins/Getrag/NP205/HP60/D70