Truck/car doesn't matter. 5.125 is early, like 1960s and earlier. The 5.125 bells are a chore to find with the butterfly pattern (most had the early pattern) and never came with hydraulic clutch. 4.8whatever is later, like 1970 on. Depending on application and age the 4.8whatever bells will have hydraulic clutch bracket. They can both house the 164 tooth flywheel that can take an 11" OEM clutch. (talking about Ford specifically here)I take it that the 5.125" centered bell housing is the TRUCK version, the smaller 4.8" is the CAR bellhousing?
Any fifference in I.D of the clutch housing?
Truck running the larger diameter plate?
What about the hydraulic throwout lever set up, my workshop manual does show a hydraulic set up, but later model than 1975.
Also, whilst I'm about it, mine has the "Hydraboost" brake assist and am thinking of using a firewall mounted booster, again later model??