I think a sight glass is useless in any application. You guys are going to have a lot of fun when it comes to adding rear a/c. I love the idea, and if somebody has the determination to do so, I applaud them. Making a/c lines is not that difficult, but it isn't much fun either.
Just a few words of advise, don't forget about the low side line that will need to be adapted to the rear evaporator, you'll have to splice it in too, but that is probably is a given for most of you. Once you have that done, then comes the real fun, how much more capacity do I have? I need 4 more ounces of oil and 1lb of refrigerant, or what? What expansion valve am I going to use back there? Maybe I ought to adapt the high side line back there so I can play with orifice tubes instead? Do I need a smaller pulley on my 10PA17C? Is there enough tensioner travel to accommodate a stock belt if I do? Is my condenser large enough to dissipate the heat? I probably need an electric fan to help cool everything down, huh?
Where I learned a/c, we did a few things of this nature. I know a few of our member's V8 FJ60's, done by a guy in the DFW area, were brought in to the shop. I had my reservations about taking a compressor that is junk, Denso 10S20F, and switching it from an orifice tube to an expansion valve (making the necessary fitting/line mods), but it wasn't cost effective to put a better compressor in place. I'd bet that most of those compressors have failed.
If anyone local wants to do this, I'll help as much as I can. Also, if you want the high side lines to relocate the drier, I have the FJ80 lines. I don't want $$, just come and get them. A new drier cost me $3, and I'll throw that in to any interested party. The lines might be a bit mangled from the wreck, but they never leaked. If they need a weld to reinforce, I know a guy.
I love doing things such as this, there's just no need, or desire to tackle the job on any of the vehicles listed below. Well, maybe on the 40.