I have 3 shafts 1-3/4" x 102" with 2 tapered couplings if anyone is looking. Did a recent repower on 1987 46" Post.
6-71 TIB 485 HP with 507A 7 degree down angle gears. Original shafts are 102 " new shafts are 108". Went from 6-71 TI 450 HP with 10200 Capital gears.
good luck, that's a lot of work for not a lot more power. Since you are changing gears, Did you consider pushing the new engine farther back?
Job is completed moved the engines 6" forwaed . Found the engines @ a great price. You are correct it was a lot of work but worth the effort.