If boom(key 4) & masts(keys 1 & 2) are the same part numbers for models 146 & 148 loaders then ask them how 148 is TOO LARGE if 146 is designed to fit. Many model 146 loaders have been installed on JD 4 cyl engine utility tractors. Back in the mid 60's - mid 80's while I was employed at a JD dealership I mounted or supervised mounting more JD frt end loaders than I care to remember and as the old saying goes "this ain't my 1st rodeo"!:laugh:
IIRC from my JD dealer employment days 148 wasn't listed to fit utility tractors but I know the 48 which is basically the same size as a 148 was listed to fit utility & rowcrop tractors. I'll bet dealer is basing their statement on the fact that JD didn't list 148 to fit or furnish mounting brackets for utility tractors. Since 146 is listed to fit utility tractors and utilize the same mast & boom 148 then I see no reason a 148 won't fit a utility tractor EXCEPT if boom cylinders were longer on 148 vs 146 which would limit how low the 148 boom could lower on the less ground clearance utility tractor.