Like LORDDIESEL said the 4bt probably isn't the best option in a full-size. I have seen it done though and it is possible. With some mods installed on it the engine will have enough power to pull the truck. They do run the FedEx and UPS trucks so they can haul some weight but they won't have any get up and go stock.
A good website to look at for the 4bt and all swaps concerning it look at http://4btswaps.com

They have tons of good information on the engine itself. If you find a p-pumped 4bt you are golden.

 

here is the problem with 4bt's. You'll find 75 6bt's to 1 4bt. Finding a ppump 4bt in a good price range is just not going to happen. People think these things are made of gold. 3500+ is the going rate for a beat up one. VE style's are over 2k.

where as good running ppump 6bt's can be had as low as 700 bucks i've found. That extra money can go a long way in parts/mods. And unless you're building a jeep or something smaller where you need the room, it's just not cost effective.

 

Oh I know they cost an arm and a leg. That is one of the reasons I went with a 5.9 for my swap. Plus I will like the extra power.

 

With the 6bt what kind of mpg would i see with a GM K20, no preference on auto or manual, whichever is the simplest, economical, and durable.

 

well my truck is a 2wd, but it's longers than yours and has a 1000lb flat bed on it and i'm getting 800km to 130 liters.

You have to remember the 6bt doesn't have to work very hard to move the truck. The 4bt you'd have to push harder.

 

In American terms that is roughly 14.6mpg. There are people that get upwards of 20+mpg on the highway. It all depends on gearing and what kind of modifications you have.
I have found this website Gear Ratio Calculator does a great job in dealing with gears and giving you a approx engine RPM for a given road speed. On the 6bt cummins your best fuel mileage will come from around 1700-2000 RPM. Over 2000 RPM and your fuel mileage will drop fast.

 

yeah my chassis cab with deck isn't light. I'm turning 2000 rpm at 65 mph with 373's and a 31 inch tall tire.

 

Fuel economy has more to do with driving style than anything. I have a P-pump 6BT Cummins, 5 speed, 4.10's, 38" tires, and weigh roughly 9000lbs empty and can get single digit fuel mileage driving like an idiot or as good as 18 mpg feathering it.

The key is finding the right combination. Stuff a 6BT in a fairly light truck with overdrive transmission and tall gears with stock rubber and stock size wheels (such as many 1st Gen Cummins Dodges) and 20+ mpg is possible every day, with 30 mpg possibly if you tailor your driving style. Lets not forget proper engine tuning too!

Keep the truck light, the tune strong but clean, the tires close to stock size (no lift!) with an overdrive transmission and gearing to net you around 1800 rpm at about 75 mph and I would think 20 mpg should be easy.

 

im 18-20 with my 500hp 12v and 35" tires with 354 gears driving it right

 

Is this all highway miles, or a ,ix?