I have 3 questions for you:

1. Would the short length of rubber fuel hose between the two larger sections of hardline(near the washer fluid resevoir) be a good (safe) place to install an inline gauge?

2. Is the upper or lower line in that location the supply line? It looks like one is supply and one is return.

3. Does anyone know for certain if that is a 5/16" or a 3/8" hose?

 

1 Yes (see picture look at the little shiney thing there, theres an aftermarket external regulator in that spot now though)

2. Its the larger line, look at the metal lines youll see the larger one easily it should be the top one but dont take my word for it, check

3. Its 3/8ths