The gears in the differential on a 1-ton are 5.14 to 1. The differential is the round section of the rear axle visible from behind the truck looking forward under the bed.
While updating the motor might improve your top speed, with the current gears the problem is you will be turning the motor real fast. With the 216 going 60 to 70 is too fast to run that motor for any length of time.
Change to a 250 and it is likely to survive longer at that speed, but it will still be turning real fast. And that remains true no matter how big or modern the replacement motor might be.
Change the rear gears to 4.11 and the 216 will have trouble moving a heavy load, but the truck will manage highway speeds more readily.
Changing to an overdrive type transmission can achive the same kind of thing. An overdrive is usually about 30% meaning if it runs 45 before it will run 60 with the OD.
Best answer is probably to both swap to 4.11 gears in the rear, and run something like a T-5 overdrive 5-speed transmission. Then put the 250 in front of that, and while you won't have the same load moving grunt it started with, it will be real drivavle when empty and able to deal with some load.
What size tires do you plan to run? The stock 19 inch wheels limit your choices. If you are running smaller wheels that does affect your tire choices and the final ratio. Shorter tires will slow the truck down.
A 1-ton has an open type driveline, so you don't have to change the transmission and driveline to make some of the changes suggested. A 250 or similar motor (230 or 292) will require a bellhousing change, and that does impact the bellhousing crossmember. Since these motors side mount instead of front mount, it might be easier to use an aftermarket transmission crossmember rather that replacing the bellhousing crossmember. But that only works if you swap to a transmission that can be mounted that way...
This whole thing can be complicated, and rather than trying to detail every option it would be less confusing if you made a few of the choices, and then asked for the relevant details.
