First, it's helpful to know your Galaxie's body type. 1966 Ford Galaxie 2-door "hardtop" (also known as Fastbacks) came in four flavors: Galaxie 500 (body-code 63B), Galaxie 500 XL (63C), Galaxie 500 7-Litre (63D) and LTD (63F).
The reason the body code is important is it allows you to cross reference applications in the Ford Master Parts (MPC) catalog which is extremely useful. When I restored my `66 Galaxie 7-Litre, I researched the front springs and made the list below. My car came with HD suspension and a 428/4-speed. What I found was that there were two types of front coil springs applicable here: C6AZ-5310-T (2345lb load rate) and C6AZ-5310-S (2235lb). Here is a breakdown of applications for each one:
C6AZ-5310-T:
- Pre-Feb 14 1966 Galaxie 500 XL with HD suspension, 352 engine and Cruise-O-Matic trans
- Pre-Feb 14 1966 Galaxie 500 XL with HD suspension, 390 engine and 4-speed Toploader or Cruise-O-Matic trans
- Pre-Feb 14 1966 Galaxie 500 XL, 7-Litre or LTD with 427 engine and 4-speed Toploader trans
- Pre-Feb 14 1966 Galaxie 500 XL or 7-Litre with HD suspension, 428 engine and 4-speed Toploader or Cruise-O-Matic trans
- Pre-Feb 14 1966 Galaxie 500 XL Police Interceptor with 428 engine and 4-speed Toploader or Cruise-O-Matic trans
- Post-Feb 14 1966 Galaxie 500 XL with 427 engine and 4-speed Toploader trans
C6AZ-5310-S:
- Pre-Feb 14 1966 Galaxie 500, XL and LTD with HD suspension, 352 engine and Cruise-O-Matic trans
- Pre-Feb 14 1966 Galaxie 500, XL and LTD with HD suspension, 390 engine and Cruise-O-Matic trans
- Pre-Feb 14 1966 Galaxie 500, XL, 7-Litre or LTD with 427 engine and 4-speed Toploader trans
- Pre-Feb 14 1966 Galaxie 500, XL, 7-Litre or LTD with HD suspension, 428 engine and 4-speed Toploader or Cruise-O-Matic trans
- Pre-Feb 14 1966 Galaxie 500, XL or LTD Police Interceptor with 428 engine and 4-speed Toploader or Cruise-O-Matic trans
- Post-Feb 14 1966 Galaxie 500, XL and LTD with HD suspension, 352 engine and Cruise-O-Matic trans
- Post-Feb 14 1966 Galaxie 500, XL and LTD with HD suspension, 390 engine and Cruise-O-Matic trans
- Post-Feb 14 1966 Galaxie 500 XL and LTD with 427 engine and 4-speed Toploader trans
- Post-Feb 14 1966 Galaxie 500, XL or LTD with HD suspension, 428 engine and 4-speed Toploader or Cruise-O-Matic trans
After I made that list, what it told me is that both the C6AZ-5310-T and C6AZ-5310-S front coil springs were used on 2-door hardtop fastbacks both before and after Feb 14, 1966. So that manufacturing date doesn't matter.
If you had a copy of the MPC, you could cross reference and see for yourself. You could also verify other stuff and frankly just about anything else that comes up. FYI you can find copies of the MPC on CD at Ebay for $25. See the link below. I have no affiliation with the seller, manufacturer or anyone else, just passing along to fellow Galaxie owners is all.
https://www.ebay.com/itm/1965-1972-...6410?_trksid=p2349526.m4383.l4275.c1#viTabs_0
By the way, I ended up getting the MOOG 8336's which have 2450lb load rate (slightly more than C6AZ-5310-T), 473lb/in spring rate and 12" unloaded height. These were perfect for my application which included PS, AC, HD suspension/shocks/anti-sway bars. These are linear springs which have equal “distance” between all the coils. I am extremely happy with them.
Insanely, they can be found on Amazon for only $52.33!!
https://www.amazon.com/Moog-8336-Coil-Spring-Set/dp/B000CORVTW
Remember, the role of coil springs (and anti-sway bars) is to maintain constant tire contact, patch attitude and chassis balance. Meanwhile, the shock absorbers role is to minimize tire force variation while ensuring there is always as much tire patch against the road surface as possible. In other words, they help the tires stick to the road.
Overall, the coil springs combine with the shocks, sway bars and tires to maximize road grip (bite, traction) in order to provide good drivability and handling balance (under-steer, push, over-steer, etc). This is what I was shooting for - good balance and handling characteristics for “fun” (and heavy-footed) driving conditions.