66 Formula S barracuda

66cuda-s answered most of your questions but here's a few more.... Yes, they make reproduction resonators for the 273 4bbl cars. I've seen a few on E-bay. There is also an NOS one somebody wants an ungodly amount for!
As for the tires/wheels, a lot will depend on how centered your rear axle is. I've seen a 2" (50mm) difference in tire clearence from side to side on some cars. If it's reasonably centered, I've put 225/70/14 and 215/60/14 tires on the stock 14 x 5 1/2 wheels with no rub as well as 9/26/14 slicks on 14 x 7 deep dish mags but I had to message the lip just a little to make sure I don't cut a tire. You can't tell from the outside as I didn't change the opening shape. There are some wheel companies out there that have (or will make) wheels up to 18" with the small bolt pattern but you'll have to take width and total diameter in consideration unless you want to cut. LH lug nuts are impossible to find in some wheel types. You may have to swap to the RH type if you use them.
Reasonable price? How high is up? I don't know what your market is for those cars in your country. I've seen restored ones here for under $20K U.S. Runner/drivers for $3-12K depending on condition and builders for a few hundred to $6k. If your deal falls through and you still want one, I have a loaded '66 Formula S builder that I'll sell for $3900.
Thanks, Mark