If you go with the factory original 65-67 Mustang Kelsey Hayes 4 piston brakes you will have no worries at all with those wheels. Can even use your original V8 drum spindles with no modifications other than bolt on the factory Ford caliper adapter.
Here is a Tread on another Forum right now. It pertains to this very factor.
We need to get this clarified as to the fit of the ORIGINAL WHEELS, Not the Repo wheels.
Dan @ Chockostang
From VMF:
The 35 year thing is just numbers to demonstrate, the wheels on about everyone Classic will be new wheels, not 35 years old.
All new custom/repo wheels will fit the 64-73 Mustang Disc brakes. 14", 15" etc.
Manufacturers haven't made a drum brake wheels for at least 35 years.
As for the 68 wheels all fitting, I for sure won't step out on that and say yes. I tell Folks, 70, and up on all the Factory wheels, 14 15-Up.
If the 68 69 GT Wheel Yes, will work fine, All the Magnums. The 64 65 66 67 wheels, Maybe, Maybe not..
If someone tells you Yes, your stock 65 wheels will without doubt fit the 4 K/H Disc brakes they are not being truthful. The 65 Factory Disc brake wheel will, the 65 Drum brake wheel will not.
If the 65 drum wheel fits the k/h four piston, then the calipers have to be the thin cast smaller body, less mass versions. Only way the drum brake wheels will fit without using unsafe spacers to space out the wheel.
Dan @ Chockostang
Both original and aftermarket 65-67styled steel wheels will fit 65-67 4 disc Kelsey Hayes disks. The problem is if you use early 65' standard steel wheels (designed for full hubcaps) made for drum brakes that do not have the ventilation holes for disks. They will not work with disc brakes as they hit the calipers.