Even loaded a slant six ram should eventually go much faster than 45mph. Do you have a tach installed. Are you sure that you have a "ford" trans? That is an odd thing to find in a dodge. If it has the expected 833od trans I might expect the truck to be faster in 3rd instead of 4th gear due to OD rpm drop on 4th gear. If the engine is far enough out of proper tune it might not have enough power to pull against that overdrive.

Sounds like it is time for basic tune-up, inspections, verify if the engine is original, identify what carb is installed, verify what the trans actually is. Just roll under, take a picture, and upload it here we can ID an 833od pretty fast. If the engine is not original there is a chance it has a mechanical valvetrain that requires adjustment, 1983 should have no maintenance hydraulic if it is original. Then there are other fun potential areas like intake leaks or stuck exhaust heat riser.

Edit, if someone has messed with the shift linkage on an 833od it is possible to have 3rd and 4th gear swapped on the shift pattern. It puts 4th on top of 3rd instead of 4th down like it should be.

ford coil an a chevy module.

That is a good swap.

First verify that your throttle is opening the carb the whole way. Start there- simple stuff first.

What is the initial timing set to? What is the timing at say, 2500 rpm?

That is a good swap.

First verify that your throttle is opening the carb the whole way. Start there- simple stuff first.

What is the initial timing set to? What is the timing at say, 2500 rpm?

It doesn't open all the way. It opens up halfway.

That would be a major problem! Maybe show us a picture of the cable setup at the carb. But without a picture I would guess you need the point where the cable housing mounts to be further towards the firewall. Farther from the carburetor linkage.

Do you have a timing light? Check it at idle, with no vacuum hooked up. Then rev it up and see if it moves up towards 30 degrees when revved up.

From the picture you have a serious problem with the throttle cable mount. You must have the cable housing mounted firmly to a bracket that holds the cable in place when you push the throttle. It appears yours is zip tied to other flexible items. You or someone who can help you will have to fabricate something solid. A cheap timing light would be a wise investment or at least borrow one.

The length of the cable is not the problem. It is how it is mounted. You can have a longer than needed cable and it can work fine. It just has to be mounted correctly and not flex.

If you don't have some fabricating skills you may be better off finding an original carb and cable set up. But finding 40 year old carbs in good shape can be a pain also. Maybe someone on here has one?