That doesn't sound right at all. My 1950 216cid, 3spd, 4.11 easily makes it to 65mph and comfortably travels at 50-60mph.
Before converting your rear end, you need to find out why your truck is so slow.
Correct diameter tires will help. When I installed 16" rims and 30" diameter tires, it made a dramatic improvement in driveability and speed.
Is your clutch slipping? Is your engine worn out? I'd do a compression test just to get some basic starting point information.
Engine, tranny and rear end are not rebuilt. Clutch is new.
Prior to new clutch, top speed was between 30 and 40mph depending on terrain. Top speed going up hill was 30.
Engine runs smoothly, doesn't burn oil, compression is around 105 on all cylinders, clutch is not slipping.
I think the tyres are probably the main problem, especially if they should be 30 inch diameter, as mine are only 27 inch
