Me either. There might be some good information there, but I can't makes heads or tails of what was written.
If he's saying it is illegal I disagree. The spring section from the GCR reads:
5.b. Springs and Shock Absorbers
1. Shock absorbers may be replaced provided that the
replacements (a) attach to the original mounting points,
and (b) are of a non-remote-reservoir design. The number
and type (e.g., tube, lever, etc.) of shock absorbers
shall be the same as stock. The interchange of gas and
hydraulic shock absorbers is permitted. External adjustments
of shock control shall be limited to two (2). No
shock absorber may be capable of adjustment while the
car is in motion.
2. MacPherson strut equipped cars may substitute struts,
and /or may use alternate inserts. Spring seat ride height
location may be altered from stock. Remote reservoir
struts and/or inserts are prohibited. External adjustments
of shock control shall be limited to two (2).
3. Springs of any origin may be used, provided they are of
the same number and type as originally fitted, i.e., coil,
leaf, torsion bar, and that they shall be installed in the original
location using the original system of attachment. The
joining of two or more coil springs by any means is prohibited.
The use of tender springs are permitted. Shackles
or spacers may be used to adjust leaf spring ride height.
Spacers, including threaded units with adjustable spring
seats, may be used with coil springs. Coil over threaded
body shock/struts are permitted.
4. Spacers or lowering blocks may be used between leaf
springs and the point(s) of attachment to the axle housing.
If he's saying that you can't change the spring ride height and that makes it illegal then I disagree with that point. According to the Ford manual, the Mustang has a "modified MacPherson strut" front suspension. Due to this, 5.b.2 states "Spring seat ride height location may be altered from stock", meaning you can raise or lower the spring seat.
Furthermore, the overall enabling statement is 5.b.3:
Spacers, including threaded units with adjustable spring
seats, may be used with coil springs.
If it says you can, then you can. This statement has no limitation or guidelines on how to carry it out. Just as was pointed out earlier that the allowance for brakes ducts just say you can use them, it doesn't indicate if they need to be welded, riveted, glued, bolted, screwed, or attached to the car with bubble gum, or, if they should be made from steel, aluminum, fiberglass, or whatever else you can dig up.