It is possible to dismantle the main shaft to separate the main part with 1-2 gear with the small part with 3-4 gear.
press and some heat; but useless for you.
looks like you juste need to replace the gtv6 parts by the 75 parts for :
the main shaft complete
the pinion gears in regard 1-2-3-4-5 that roll on the bushes that roll on the second shaft.
but keep the secondary/pinioned shaft from the gtv6, in that way you dont need to reset the pinion/crownwheel distance.
Of course all this is a kind of salad mix of different parts age.
sometimes the whole thing will explode earlier than foreseen.
keep in mind that the rolling bushes (clear metal with the oil hole, not the copper ones i mean) that are beetwen the 2nd shaft and the gears had a certain thickness that do in the total thickness of parts to set the pinion/crownwheel distance.
if you keep the old gtv6 ones, the things should works fine (magnifier eyes look and dimensions checking).
but if they are 'tired' you will need to replace with the one from the 75 gearbox then clearence issue must be sorted in that another case.
---Did you are doin this combo to keep secondary shaft for its standart tachometer fitment ? you got only spare parts ? keeping the old way shifting lever (thats works well if no play anywhere...)
some picture on these pages :
viewtopic.php?f=9&t=1702&st=0&sk=t&sd=a