Wiring a JL w7

Here's what I have: one (soon to be two) jl 12 w7 and an old school us amps 400x.

Amp specs are: 400 x 2 @ 4ohms or 750 x 2 @2 ohms or 1500 x 1 @ 2ohms (under rated, but anyway)

The jl is a single v.c. @ 3 ohms, so what is my best wiring configuration for now (1 speaker) and later (2 speakers)

Well as it is a SVC speaker, there is only one wiring configuration possible.  As far as connecting to that amp, you may be in trouble as a US Amps 400X is only rated to 4-ohms when bridged.  It will probably handle a 3-ohm load, so I'd try that.  Bridge the amp with the one speaker and see if it remains stable.  When you get two speakers, you'll have to wire them in series for a 6-ohm load.

i don't know where you got the specs you list.  According to the manufacturer, that amp is rated as follows:

You may want to look into a different amp.

So how do I wire two of them?

You've already answered your own question.  Didn't you say above that you ran this amp at 1 ohm with no problem?  1.5 ohms is a lighter load, so....

You know there is a wiring configurarion application in the left column, right?  1.5 ohms or 6 ohms, take your pick.