In approximately 10,000 miles I've had no issues with a modded 90 S10 T5 input shaft. I shortened the shaft length and the throwout bearing collar to match the original SM420 parts. To address the clutch spline length issue, the important change was using the bench grinder to reduce the input shaft diameter (inboard of the splines) so it matched the diameter at the bottom of the splines for (as I recall) about 1/2". This allowed the clutch disk to not bind due to the splines being too short on the now shortened shaft.
Would I do the above mods with a V8 or high HP 6 cylinder powered truck? NO SIR. But my truck makes 85HP. Even with a higher HP engine, I wouldn't expect any problems since I don't do smokey burnouts or WOT driving these days. Well, not with the '51 anyway.