450 Transmission swap

[MyJD450]

I have a JD450 dozer, approximately a 1970 build, and am having transmission trouble. Repair shop hasn't diagnosed yet but wanting to get a head start in case I need another tranny. Repair shop recommended changing Constant Mesh Transmission out for the H-L-R tranny if another one is warranted.

My question is- Can a H-L-R tranny be substituted for a Constant Mesh Transmission, and if so, are there many modifications to make or hurdles to go over?

Thanks