Most Hewlands behind small(er) engines are reliable to the point of boredom. But once you exceed the design input levels of torque and rpm, things tend to go downhill quickly.
The "rhino" case is helping "elevate" the max torque the box can handle. I'm thinking you "might" be close to the edge though. Checking ring and pinion wear should be a regular inspection. Use a small inspection camera inserted through the original drain plug, now on top of the R & P section. You might have to drain the gear lube. Genuine Hewland R & P sets hold up the best, if they are still available. We had lots of problems with Mk 9's behind YC's, until we upgraded to "rhino" cases. They were then, just acceptable. The drivers were not the easiest on the gearboxes though . . . . . .
And 1st and 2nd gears might be an issue at elevated torque input. I'd run the integrated shaft 1st gears only, and I would run as tall a 2nd gear as the car will stand. 2nd gears tend to break in the small section(s) between the bottom of the gear teeth and the tops of the shaft splines where the gears are broached for the splines of the 1st gear shaft. The small section there tends to be weak and can not withstand excessive torque for its' size. Taller gear ratios are thicker there. You will want to look carefully at both 1st & 2nd gears, anytime the gearbox is apart. A tipoff is: if the 2nd gear becomes difficult to "slide off the 1st gear shaft", it is probably distorted, and should be discarded. Every "distorted" gear I've ever seen, has failed "magnaflux". The 1st gear shaft can become "twisted" as well. The tipoff is that the other gears, (2nd, 3rd, etc) will not slide easily on the shaft splines. That is the discard point.
And I think it is important to note that gearbox "wear" varies with a driver's "driving style". The old Indy joke was that Rick Mears never broke gearbox pieces, but Mario Andretti could break a Hewland just by walking by it . . . . . .
YMMV
Fordboy