Okay here's the deal:

Ive been reading all day about firstly upgrading to a disc rear, no one seems to have a consensus about using the master cylinder from the donor car, some say yes you need it some say no. Here's my plan Im getting ahold of a disc rear from an 89 iroc, i have an 89 RS with a t-5, i read that i need to grab the ebrake cables and the proportioning valve as well. I also read that the gear ratio's may be diffrent thus affecting my speedo, and i may have to change something, is this the case even if i have a iroc 140 mph speedo. I dont know what my ratio is now, i read that a lower one might make my car slower or harder to drive, what is the ideal ratio for my 305 TBI and t-5 ?

Now secondly i plan on upgrading to the LS1 style brakes for the front, i see that i can buy a kit from bigbrakeupgrade.com for 600ish bucks, i read it's very worth what there asking, and install is pretty easy. Now id also like to do a little upgrading for the rear after it's in, can i put on some 4th gen parts on the 3rd gen rear ? Suggestions/problems with this plan ? Thanks fellas !

I dont want to get a 4th gen rear because i plan on running irocs for the winter and my Z06's during the summer W/ adapters

 

From Dan Burke's page:
How to Upgrade to 1LE brakes

Master Cylinder: p/n: 18014286 - Same as the J50 option drum brake equipped cars from 1984-up. If you have drum brakes, you have the right master cylinder! It is only us poor schmoes with 1982-1988 rear disk equipped cars that need a different master cylinder.

1LE Combo Valve (proportioning valve)
14089496 1989 1st design: M1.0 tube nut threads (For 1982 - early 1989 applications)
10136840 1989 2nd design: M1.5 tube nut threads (Late 1989)
10136840 1990-'92: M1.5 tube nut threads.

You'll need the e-brake cables.

That your speedo is a 145 makes no difference. Depending on the gear ratio of the new rear end, you may have to change the drive/driven gears in the tranny.

JamesC