#1
Okay so I love the look and size of the vintage fender 2x12 guitar cabinets. Is it possible to get an empty one and put two 12" Ampeg bass speakers and wire it to Ampeg specs? If so, do wires make a difference and will the Ampeg speakers fit in the cab depth wise?
#2
Well; depending on your carpentry skills, you can build just about anything. Wiring it to "Ampeg Specs" should be no problem. Yes, the gauge of wire that you use will make a difference in some respects, but you need to speak to an electrician to make sure. You might want to post this in the Gear Building Forum. They can probably answer such esoteric technical questions for you.
"Maybe this world is another planet's hell?" - Aldous Huxley
#3
It's not a foregone conclusion but many, if not most, bass speakers won't physically fit into guitar cabs. If you look at bass cabinets they are usually much deeper than guitar cabs. So... you probably wouldn't want to build a copy of a Fender guitar cab.

Then, there's the issue of how good it will sound even if you find bass speakers to fit. Bass cabinets must be "tuned" in order to deliver premium sound. You will find most bass cabinets are designed with specifically engineered ports. Others (like some Ampeg cabs) are sealed units. Since many guitar cabs are open-backed installing bass speakers without regard for tuned porting can easily result in a crappy sound. And, if they're sealed units there's not enough internal volume to allow the air to move as it needs to.

It is entirely possible to start from scratch and build a bass cabinet to install bass speakers in, but it requires calculating numerous design parameters to end up with a really good sounding cab. Most amateur designers rely on Win ISD software to calculate all the factors (internal volume, speaker excursion, porting, etc.).

IMHO you would be much better off, both financially and effort-wise, to simply buy the bass cabinet that best suits your ears.