This is from NMCA NSS rule book. FX Class allows any period correct wheelbase altercation. However the front suspension doesn't say anything about allowing straight axles. I would say call NMCA to see what they say. I know that 422 Motorsports allows AWB cars to run NSS... at least at the Beaver Springs race they do.
3.4 SUSPENSION
FRONT SUSPENSION: Stock front suspension required. Aftermarket direct replacement
suspension components including A-arms permitted. Aftermarket, direct replacement bolt
in, tubular A-arms permitted.
REAR SUSPENSION: Rear suspension of ladder bar, three/four link, or stock rear
suspensions permitted.
4.12 WHEELBASE
WHEELBASE (NSS CLASSES): Must retain stock wheelbase with a maximum
variation from left to right and front to back of 1 inch. Stock overhangs are required.
WHEELBASE (FX Classes): Any period-style alteration of wheelbase and overhang is
allowed.
AAAA\FX 8.50
AAA/FX 8.75
AA/FX 9.00
A/FX 9.25
B/FX 9.50
C/FX 9.75