[DieselSJ]

I have driven a 4bt. They are not smooth, but they are not horrendous rattle-traps either. Liquid filled engine mounts are key.

Top speed is all about gearing. You want it to spin around 1600-1800 rpm on the highway. My diesel Jeep has 28" tires with 3.31 gears and 700R4 and it sits about where I want it on the highway and will do just over 100mph. What RPM is your current setup giving you on the highway?

Faster than a 454? Which 454? What mods to the 4bt? Faster as in top speed or faster as in can beat it in a stoplight to stoplight race?

4bt over a 454 - A 4bt will get you 20mpg plus with the right gearing. A 4bt can be built to generate a reliable 500ftlbs of torque and still retain nice manners. A 454 can also be built to do that, but it will drink probably twice as much as the 4bt.

IMHO a stock 4bt will be horribly disappointing, whereas one with pump mods, injector upgrade, valve spring upgrade, turbo upgrade would be a great engine for a 1/2 ton truck that occasionally pulls a 5k-lb trailer. Do not assume that you are going to drop in a stock 4bt and be happy - you won't be.

Why not find a 6.5 drivetrain from a late 90's truck and drop it in? I've got mine tuned to put out about 500ftlbs of torque and I get 24mpg on the freeway in a full-sized Jeep. That would literally bolt in to your truck.

Also, stop thinking about "speed" and "fast" with a diesel. You drive a diesel to pull things, not go fast. You want speed and fast and tow, do a mild 454. You want to tow and get 20mpg, then do the diesel.