Temporary Removal of A/C Compressor

Good afternoon, gang.

I am unable to post pictures now, but I certainly can tomorrow if not this evening. In the mean time, I'll present the question(s) just to introduce the thread.

First, the car in question is a 1974 Plymouth Duster with the 3.7L 225, automatic. I am doing some deep cleaning in the engine bay and my a/c compressor is nearly good for nothing (very old, no Freon, bad clutch, etc). I plan on removing it temporarily (brackets and all) with hopes of replacing the whole set-up at a later date. My belt configuration is good to go without the compressor present. It has been running fine without the compressor functioning. In the process of removing the lines leading to the compressor, I noticed a line running towards a cannister connected to the transmission oil cooler (or based on my research in the '73 and '74 manuals, that's what it is indicating). I can't find the intention of this line, and I am afraid to remove it in case there is a function that would either: A. harm the function of the engine while running, or B. harm me in the process (I doubt there is anything of extreme harm in there, but in the off chance there is something under pressure, I don't want to risk it). What is the purpose of this line? *pictures to come later*

Second, once I get all of the lines removed, would I simply plug the hole on the firewall and on the cannister? What would you recommend? My first thought is to get a threaded cap nut (or something in that family) to seal it for later.

Thanks for all the help. I'm really learning a lot from you guys.
Zach