I saw an article in a mag recently that said a 1969-73 gas tank would drop right in, inplace of the stock one in 1967 stangs??? The bonus is it is 20 or 22 gallons.
You apparently need to bend the filler neck on tank a little???
I've done this before on a couple of '67's. Just watch out for the 69/70 gas tank with the evaporative recovery system. As your 67 doesn't have that, you'd need to seal it off. But just ask when you buy your new tank, get the one w/o it and bolt it in.
My understanding is that the filler neck has to be cut down a little or bent some. But its not like it flows air..as long as the gas can get through it you are fine. Have considered doing this for years.