Hey! I just had a thought ...uh oh, bear with me.

Option 1: machine a wheel adapter on your own if you have the skill
Option 2: buy one from a reputable wheel adapter manufacture
Option 3: (my thought) the Wilwood brake kit I used on my old '68 came with new hubs that you had to assemble yourself--and they were dual patterned. You could choose to go with a Ford pattern or a Chevy pattern. The only add about 1/4" outward extension of the wheel hub mounting face. Problem solved.

Of course...you have to pony up the cost of the brake kit ($650 or so), but hey, they're great brakes!

Just a thought...

 

my idea was to machine the wheel adaptors myself and use a 4.75 bolt circle to bolt it to the rotor/hub on the car and a 4.5 stud pattern for the wheel.What I need to know is the stock mustang wheel offset so I can compare that to the stock corvette rim offset in order to come up with a thickness for my adaptor.Vette Brakes and Suspension sells a similar adaptor for bolting late model vette rims that have more offset to c3 vettes.

 

I agree with all you have said in principal but the OEM 17x8 bullits have a 45mm positive offset. (6.295" backspacing); the 18x8.5s have the same 6.295" backspacing so they will stick outward 1/2" more; meaning they have 1/2" less positive offset or 45mm-13mm or approx 32mm. FRPP tech should have exact offset for you.

45mm on the OEM bullits is a lot of offset; imagine a 8" rim with 6.295" backspacing. That's why folks with aftermarket rims on the 05-06 all stick out; a little, to some cases quite a bit. So in that sense because most of this rim is well inward of the bolt face, adapters would seem to be no problem at least as far as wheel location is. ...which is what you were saying. ...no disagreement there.

Except for the price I can't see the motivation but I guess that's enough; have to mod the 'Ford' logo centercap but that's no problem.

 

45mm on the cheap silver bullits you are thinking about.