8 3/4 rear 741 / 742 center case question - HELP!

Hi there. I am currently running a 8 3/4 rear with an open 3:55 in a 741 case. When i got the car it came with a 3:91 sure grip in a 742 case.

What is the difference between the 741 and the 742? Will the 742 centre just bolt in?

I want to install the sure grip but don't want to run the 3:91. As i do mainly cruzing i was considering somthing a fair bit taller so 60mph would equal around 2,500 RPM max. Any suggestions on ratios?

Appreciate any advice.