Well, the Caddy parts supposedly don't add any more rotating mass, so the stress to the bearings and rings will not change. That leaves rod stress. The newer rods are probably stronger than the old cast 261 rods, so you can probably rev to 4500 without undue stress. The stock 261, geared right, will last just as long as a 235 if it is not abused, so I would not hesitate to take it on any road trips. If you balance everything perfectly and keep the mixture under control and run a good oil filter you should be able to outlive a stock 235 easily. The 261 block was specifically designed for trucks and busses and can take a lot of heat abuse. The castings are thick and the cooling passages are good. I would run a 261 bypass thermostat in it but it should not complain on long trips.
I bet the caddified 261 really pulls some good torque and you could get away with 3.08 gears for cruising with good power.
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1950 Chevy pickup with '62 261, 4 speed.
