Joe:

Several years ago I saw a PV with air conditioning where the evaporator was mounted inside the heater core box, allowing fresh air into the system and using the car's heater fan to force air, rather than add on a big unit under the dash, where there isn't much space, especially if you want to keep the stock hood latch.

I have the block bosses for mounting an A/C compressor ro my B20, and I could definitely use AC out here. I'm looking at a Datsun pickup truck system, which is very compact to begin with.

I haven't seen the idler pulley you need, but I'm sure someone here nows the part you need and can fix you up.