AC question
 

My 39 studie is coming along. My builder says he needs an AC low pressure shut off valve....would that be the one that goes on the dryer or the other one?

Dryer.

I'm not aware of any other cutoff on our cars.

There is another listed above in parts for sale described only as a 400 psi valve.

Many of these switches have both low and high pressure limits. That is true of the switch in my 1984 & 85 300D, which wouldn't be a bad choice for you since it has simply two 1/4" spade connectors ("57 terminal"), though might be smarter to use a Chevy or Toyota PN. The switch-off levels aren't particularly critical, say <10 psig or >300 psig. When charging the system, you usually want to bypass the switch so the compressor will run, though sometimes just adding the 1st can gets it above the 10 psig cut-off. When I put a Sanden compressor in my 1985 300D, I added a little "pressure relief valve" ($8 ebay) on the head (rather than just plug that port). I recall it pops ~300 psig. The factory Harrison R4 compressors have a cir-clipped port, but I think it is just a plug in our cars. I recall someone said that GM put a PRV there in their cars that used an R4. I think the EPA frowns on any PRV today (releasing any refrigerant ist verbotim), though the HC refrigerant I use is no worse for the earth than a cow fart.