Sam,

I just did another one of these this week.  Pretty basic...

-I always replace the receiver/drier, expansion valve and all O-rings.

-Pull the compressor and let it drain for about 30 minutes.

-If the system has been open for any length of time you should flush the
system with A/C cleaning fluid.

-Re-assemble everything with new O-rings and liberal amounts of esther oil.

-Evacuate the system for at least 30 minutes, make note of the readings on
the gauges.

-Let it sit for at least 30 minutes, check the gauges to make sure it held
the vacuum (i.e., the readings didn't change).

-Add freon till the high side reads ~250psi with the A/C on and the engine
running at ~2000RPM.  Low side pressure should be ~35psi.

-Turn off the A/C and let it sit for a few minutes, the high and low side
pressures should equalize.

-Button it all back up and drive down the road with the windows up! ;-)

BTW & IMHO, the downside to R-134 (other than its inferior cooling ability)
is the amount of power it robs from the engine when compare to R-12.  An
R-12 system operates at a lower pressure than the R-134 system, which means
the engine does not have to work as hard to turn the compressor.  The vans
I've converted had a noticeably less power with R-134 than they had with
R-12.  Perhaps it's just me...

Cheers,

Jeff