i am planning on converting the a/c system to 134a. the body shop can recover the r12 and i was figuring to replace the accumulator and orifice tube. has anyone done this process and what is your opinion of the change. 

I did a bunch of the conversions (not on Vettes) years ago when R134 first came on the scene and R-12 was getting pricey and hard to get. Some folks say the R134 doesn't cool as well but I usually had 42-45* vent temp idling in the shop after the conversion-- which IMO is good.If your compressor is in good working order  now w/R-12---you'll be fine w/R134. The retrofit kit should come with the correct PAG oil which is what you'll need when changing to R134 in a system which used the R-12 mineral oil.

I've swapped a ton of GM systems from R12 to 134. Most of them worked just fine, and cooled just as well as the R12. The couple of vehicles that just didn't seem to cool as well were both very large. One was a Fleetwood Cadderack, and the other is my 'Burban w/rear air. The 'Burban will eventually cool down comfortably...it just takes a while to cool off the huge expanse of free space.
Everything else I've converted has cooled really well with 134.
Change the attumulator, the compressor oil, and the orifice, and you should be good to go. You really need to remove the compressor and drain as much oil out of it as you can...that's where a lot of the oil will be. Then put 1/2 of the new PAG/Ester oil in the comp., and half in the 'tumulator. Evacuate, charge, and chill.

Bought "The Beast" in the fall of the year so we never really had a chance to really test the A/C on.  Before heading out on a two week road trip the next spring had the system checked out and discovered the condensor was shot.  So had the conversion done at the same time.  IMO the A/C just isn't as cold as it could be.  But then the ZR-1 has a slightly smaller compressor installed so it wouldn't steal so much horsepower.

heyv jim, how have you been. i just found this out the other day. by any chance did they overfill the system as putting in too much 134a will not make it as cold. seems like everyone i talk to says its as cold.

I just changed over to 134a myself. My compressor was shot, so I figured now was the time to do it. I changed out all of the o-rings, orifice tube, accumulator and compressor. I didn't have the necessary guages and vacuum pump to fill the system so I took it to my mechanic and had him flush the entire system as well, he said it wasn't really needed but I wanted it done for my own peace of mind. After the charge, the system is cooling great. VERY COLD air blowing. More from the defrost vent than the dash vents. I need to tear apart the dash and clean or repair the vacuum canister that controls that. But for now, it works and with the 100+ degree days we've been having...gotta have ice in the cabin!