difference between a 742 and a 489?

How much power are you going to put to it?

What gear ratio do you want?

There are more gear sets for the 489 case than a 741

I have 3 741 cases. They were in the small block darts with 340s and held up well. But now try and get a set of 3:91 gears, not available

Had a set of 3.73 put in my 741 case and they whine so bad think they are coming back out and a factory set of 3.55 in a 489 case going in or the factory 3.23 in a 741

Read the 742 case was used in the big block Darts, don't know what else used them but probably b body cars

If you have a choice pick the gear ratio you want in a good quality gear, then a case it will fit in.