ST10 transmission swap

Hello.I'm in the process of restoring an ST10 and thinking about
replacing the 10HP Tecumseh with a 12.5HP CCKA twin Onan.
This engine has a 2 gallon oil sump and is all cast iron.The
tractor has the 6 speed in it,but I'm thinking of replacing it with
a cast iron 4 speed from a 1968 Sears 8 HP suburban.The 10
HP is a 1976 model.Both tranny's are in excellent condition.Which
one is considered the "strongest" to handle the increase in torque
and HP.Thanks for any suggestions or help.

Well, the 6 speed tranny in there now is the same one used in the later Sears GT19.9 tractor. With the twin Onan in those, the amount of torque would be near double the Onan you plan to use. No problems there at all.

I really can't comment on the strength of the 4 speeds......I've only had one and it was a 1965 Custom 6....not exactly a torque powerhouse.

not sure about the trany deal . but i have a 66 custom 6 with the four speed and a tecumseh 10 hp on it and i have no problem pulling my big trailer that weighs about a 800 lbs. i can pull it in 4th gear without any problems

All 6-speed transmissions are not the same. I swapped my 12 horse Tecumseh for an 18 horse OHV Tecumseh and the transmission soon showed signs of distress. I replaced it with a transmission out of a 16 horse parts tractor I had purchased and found the previous owner had not stored anything properly and the transmission probably had some water condensation in it and a bearing went. I decided to rebuild the newer transmission using any good parts I could salvage out of my old one. I found the gears and some other internal bits from the 16 hp transmission were much heavier than those from my old 12 hp model. Your 10 hp 6-speed will be much more likely to accept the torque from the Onan than anything out of an 8 hp tractor.

The design engineers did not specify anything heavier than it had to be.