The 88-92 G series transmission is built and mounted the same way the H type transmission is (in smaller form). Overbuilt compared to later models.
Over the years, of all the GT conversions and FE3 builds using the G series transmission I have never heard of the casing cracking or breaking. The failure is either a shredded diff or stripped gears.
The mounting bracket to transmission surface/bolt points on the later model mounts got smaller but the layout stayed the same, probably using thinner castings resulting in more unsupported surface will less material holding it together.
Ademan ran or runs a G series transmission on one of his builds and I'm sure the N/A on Nos was also on a Gbox.
The only issue I could see having with the G series casing would be at the rear lower mount, unlike the H type mount that's two piece the G mount is one piece and cradles the transmission. If the bolts holding the mount to transmission get loose with the two piece mount it just binds but the one piece mount would hold the bolts straight putting the load from any movement directly on the aluminum mounting loops in the casing. Some locktite and enough torque on the bolts to make sure the bracket is holding the transmission instead of the bolts would eliminate that possibility.
If you weld the diff shaft or avoid burnouts and heavy acceleration on one wheel (accelerating leaving a gravel shoulder, sharp turns and heavy gas at low speeds...), run clean quality gear oil. 3RD gear would remain your weakest link and it's not a matter of horsepower that will strip it, it's torque that will kill the transmission, every transmission I've destroyed was right after shifting under heavy acceleration, falling in the highest torque part of the power band in the next gear.
Personally I think it's torque difference and not overall torque output that kills the transmission, going from 2nd to 3rd on a set-up that spins in second and boosts more in 3rd will put a greater sudden load on the transmission then a well balance and tuned car putting out higher torque and power numbers but building it evenly across the power band and gears. A transmission that can handle 400ft-lb torque can probably be killed by suddenly dropping 200ft-lb torque through it, either 0-200ft lb or 100-300ft-lb instant load would be far more shock to the gear and diff then gradually building 400ft-lb torque ever would. Honda transmissions run on motor oil and are made of paper. With the low torque, high rpm honda engines you can make 500+hp but never see 325Ft-lb torque. An F2T can put down 325Ft-lb torque and as little as 250hp.