Re: C4 or 4 speed???
Single rail will last behind a stock 351, I've had two 351 in different vehicles and they lasted OK. Generally the first thing to go are the bearing, you'll hear them whine, the input shaft will wobble a fair bit.
If your spinning the wheels a lot and use the gearbox agressively it won't last as long.
The RPM at 100km/h for both a c4 and single rail would depend on your diff ratio and tire size. A 2.75 diff with 14' tires will do around 2300rpm at 100.
Manuals are more fun than auto's I'd go the manual single rail, then when you have more $$$$ buy a toploader, you should be able to get one for around $500-600.
Also don't use a twin plate clutch, use a single 11' clutch and pressure plate, there stronger.
You might have to change your diff and ratio to handle 351, whats the current setup?