One would assume that if your split case is the later variety ( 38mm idler ) that you would be able to swap the input gears but I really don't know. I thought the H55 and late H42 had the same spline count and length because you can swap in a H55 using a stock US split case ( 2.27, with either 34 or 38mm idler ) as long as you add the oiler cup.
As far as I know, the only differences in split cases were the 34 vs 38mm idler gears and a 2.27 vs 1.98 low reduction. The 38mm idler was implemented in 6/86 ( I believe ) and all the split cases mated to the H55 came with the 1.98 ratio. All the H42 split cases had the 2.27 ratios, but I'm not sure about the H41's.
As far as I know, the only differences in split cases were the 34 vs 38mm idler gears and a 2.27 vs 1.98 low reduction. The 38mm idler was implemented in 6/86 ( I believe ) and all the split cases mated to the H55 came with the 1.98 ratio. All the H42 split cases had the 2.27 ratios, but I'm not sure about the H41's.