Filter access is a big reason for it. For those of us running header instead of exhaust manifold, changing the filter ends in oil on the pipes that you can't clean out completely.......the smoke is awful.
Try Performance Auto Transmission Center
A500/500/42RE/44RE/42RH/44RH Archives
They might have the solenoid plug. The neutral safety switch connector should be available on Rockauto still.
Salvage yards have them for a decent price, usually. The aftermarket ones are hit or miss for quality. You can search the part number online, and find a factory one from a dealer that sells at a discount.
I think the spring stays the same, you could probably make the shims to adjust the pressure, using the Borgeson instructions and measurements off of the shim that is already in the valve
ATF+4 is the current recommendation, it plays well with the older fluids, and has very good characteristics. The latest Dexron will work as well. Type-F is good in a hi-performance build, but is an old fluid that lockup torque converters don't like.
They are right, I had one in a trans years ago, I remember trying to figure out why it didn't match the spare drum I had. My Uncle (had a transmission business) was confused by it and gave me a correct 5 disc for cheap.