If you want to change your engine and it is the original with the numbers on it, most trucks that age have had the engine replaced and the numbers are not original anyway, you can go to the DMV and have the cab number verified along with the orginal engine number. Explain to them you are going to replace the engine and want the truck registered by the cab number. They will probably want to see a bill of sale for the new engine.

Almost every old truck I have had, I have been through this because the engine either was not original or there were mistakes with the numbers.

Not a big deal.

To get the speeds you want, you should change the rear end along with the engine. The engine will be happier at a lower RPM and you will not think your engine is about to blowup when you are trying to get out of the way of a big rig on the freeway.

54 Chevy build pics.