I shifted to R134a last summer in my 86 Syncro.  It kept me plenty cool in
95 F with a dewpoint of 75 F at Buses by the Arch last summer.

Of course it is essential to eliminate all outside air flow with any a/c
system.  Letting in outside air gives you hot air, and worse, fresh humid
air, all of which has to be cooled and dehumidified by the a/c system.  Much
of the energy in cooling goes to removing the humidity, so if you can keep
fresh air out of the van, you will recirculate air that is progressively
getting colder and dryer, which makes the system more efficient.  Naturally,
there is always a small bit of air leakage, but the goal is to reduce it to
a minimum.

Additionally, my a/c guru said that for longevity of the components, it is
essential to replace the receiver dryer when shifting to R134a.  This allows
almost complete removal of the old oil, which can then be replaced by ester
oil.  I am not an a/c expert, but I trust those who are, especially when I
can drive around and keep plenty cool.

Budd Premack
86 Syncro, 84 GL(for sale soon), 73 Super
Minneapolis, MN (Land of Sky Blue Waters)