[argonaut]

On what road can you legally and/or safely "cruise" at 90mph (in a 50 year old rusty truck)?

No offense, but I think you are proving your username to be true...

Regardless, to answer your question, it all depends upon gearing. You could put a 2.73:1 gearset in your rear diff, and an overdrive transmission, and probably "cruise" your truck at 90mph on the highway all day.

I haven't done a 6BT cummins swap myself, but from what I've seen it is a very long and very heavy motor. SO it is a litle more involved than just some new motor mount brackets and one wire. You're talking suspension upgrade, modification to the firewall, fabrication of a front crossmember with motor mounts, exhaust, relocating the transmission and transfer case backward, getting driveshafts shortened/lengthened, etc.
I have seen people fit 4BTs in quite nicely.