If now all the manufacturers could get along and decide a scale, I would be rather glad to be honest. After all, a vehicle sets you back 20-30 quid, and that's quiet an investment in the end.
Well we know that is not going to happen and if it did, you'd have to convince a significant proportion of wargamers that whatever they chose was the right scale. As it is there is a significant portion that believe that the 28mm 'average soldier' is 160cm or 5'2" tall (1/65) and not 177cm or 5'10" tall (1/55-1/56) and that real people measure themselves from foot to eye, so that a 'heroic' 28mm (foot to eye) figure is actually likely to be 30-32mm tall, with 'true 28s' being around 25mm foot to eye (about 1/63).
At least with manufacturers working to different scales there's a sort of lottery effect were you have a chance of matching a specific scale of vehicle to a specific size of figure. Of course if they went for 1/60 scale vehicles, then everyone could be equally unhappy, which is about as good a consensus as you will ever get.

The vast majority of photos of US M48A3s that I've seen show the M1 machine gun cupola. Sometimes the .50 cal was removed and mounted externally but this seems to have been the standard fit. At any rate it wouldn't have been too hard to have given the modeller a choice of cupola or hatch.
Indeed you won't find an M-48A3 in Vietnam or in the US Army without an M1 cupola and it was retro-fitted to the earlier designs, barring a few training vehicles... seeing as it features on the box art, you have to wonder why they left it off the kit.