You can get a "lighter" replacement spring from Steeda (recently released).
To get actual FORD parts, you need to buy the whole clutch & brake pedal assembly as the clutch assist spring and related parts are NOT AVAILABLE SEPARATELY.
Maybe you want to look harder and try to find your original parts !!
My Stangs ...
1969 Calypso Coral 351W 4V 4-Spd 3.50 T-Lok Deluxe GT Hardtop (owned since '81)
2016 Guard 2.3L Ecoboost 6-Spd MT Base Fastback, 641, 66W, 13D, 47AX O-T-T Stripes (ord 2/11 ~ del 5/23 ~ mine 5/26)
2011 Kona Blue 3.7L V6 6-Spd MT Prem Coupe, 203A MCA & HID's (0-129k mi)
2004 Comp Orange, 1998 Chrome Yellow & 1994 Bright Blue V6 5-Spd Coupes (0-126k, 131k & 89k mi)
1979 Fairmont Futura w/ '85 HO 5.0L, built AOD, 3.27 T-Lok + more (a Stang w/real back seat & trunk)