For racing, I always thought the 742 was best but there are many opinions out there. a 742 didn't use the one-time crush sleeve. As for the 741 that is still a strong pinion. I have a friend with 3:91s on in a 741 case behind a 440-6 motor. If you are not running hot sticky slicks at a race track starting line, you'll be okay. If you are, I would get a 9-3/4. I broke an 8-3/4 with a very light car (2300#) at the starting line.
If you can get a complete, setup , posi style, new gear set, center section, like Carl got from Randy's Ring and Pinion, by all means get one.
It might save you money if you don't have to buy a housing, then the gears, then pay for setup, etc, etc......