Need to replace booster and master cylinder, No code number on parts sticker . 83 Suburban, 2WD, auto, 350, Replacements say JD5 or JB7 but how do I find out which is correct?

 

With the vin number the dealer should be able to tell which one it takes.

 

AC Delco 18M217 Master cylinder as to your B-B
Should be in your glove box, but here are the numbers:

JB5 BRAKE POWER, DISC/DRUM, 6,400 LBS
JB6 BRAKE POWER, DISC/DRUM, 7,200 LBS
JB7 BRAKE POWER, DISC/DRUM, 8,400 LBS
JB8 BRAKE POWER, DISC/DRUM, 10,000 LBS

Similarly, here's the list of brake systems that used the hydroboost:

JD5 BRAKE DUAL POWER, DISC/DRUM, 6,400 LBS
JD6 BRAKE HYD POWER, DISC/DRUM, 7,200 LBS
JD7 BRAKE HYD POWER, DISC/DRUM, 8,400 LBS