i've never gone "guage shopping" at a local parts store but i think you should be able to find a mechanical guage with a direct connection with threads. otherwise i think there are little kits that let you hook a hose up to it. you should at least be able to order one.

but i have no idea what the thread size is.

i'm sure one of more experienced will come in here and shine some more light on this issue for you later :)