Try rockauto.
For grins, I looked up a flywheel for a 74 maverick
with a 302. A 157 tooth flywheel, 10" clutch disk, with the emblance you need cost about 91 chickens....

Worth noting that the number on it predates the mustang.  C3 pegs it as a part originally made in '63.

I would advise just getting 157 tooth flywheel with 28oz. imbalance drilled for a 10.5" clutch.  This is the easiest/simplest route to your T5 swap.  This is the one I run on my 331:

https://www.summitracing.com/parts/fms-m-6375-a302b

Yup, 160. During my recent search for a Henry made flywheel I counted 160 teeth on one. Wtfresno? 157 and 164 for Windsor motors was all I'd ever heard about. Counted it 2 more times to confirm.

The thing that would worry me is whether or not the 160 toother would fit in the T5 bell.  There isn't much clearance in there for the 157. 

To my knowledge the 160 tooth flywheel was being used on Hi-Po 289's, and maybe other applications.
The Lakewood scattershield I purchased back in early 70's came with two starter upper bolt positions to accommodate both the 157 and 160 tooth flywheels. 
I always wondered why Ford had both when both a 10.5" and 11" clutch plate fit both flywheels ... maybe for the additional weight (?)

Texas! wrote:

The small diameter flywheel/flexplate originally had 160 teeth but they were prone to breaking so Ford reduced the number of teeth to 157 and made the teeth slightly larger. They are fully interchangeable with each other.
The large diameter flywheel/flexplate has 164 teeth. It is not interchangeable with the 157/160 tooth flywheel/flexplate.
 

So the diameter is the same then?  What about starters?

TKOPerformance wrote:

Texas! wrote:

The small diameter flywheel/flexplate originally had 160 teeth but they were prone to breaking so Ford reduced the number of teeth to 157 and made the teeth slightly larger. They are fully interchangeable with each other.
The large diameter flywheel/flexplate has 164 teeth. It is not interchangeable with the 157/160 tooth flywheel/flexplate.
 

So the diameter is the same then?  What about starters?

The 157 & 160 tooth manual transmission flywheels used back in the 60's/70's were different diameters, hence the two mounting locations on the scattershield I purchased.  The starters were the same.  When I purchased the scattershield, I also purchased the block plate.  Unfortunately, the block plate I got was for the 157 tooth flywheel and the starter drive gear hit the flywheel and did not engage the flywheel teeth.  Fortunately, the engine was on an engine stand and not in the car yet, so changing to the correct block plate was easier. 
This is "vintage" information, I do not know if things changed through the years after the 70's.
 

so I am only guessing here, but here goes. while bolting the t5 bell housing to the 74, 302w block, and adding the t5. a new fly wheel which has two weights that can be bolted on depending on what balance your application has 28 oz or 50oz which in my case is the 28oz., is what i'm doing.  I can later put the 89, 5.0 in my car and that same fly wheel will work just by changing the bolt on weight balance. since it is 157 tooth (lightened fly wheel) and again it's going in the t5 bell housing I should be able to use the 89 manual starter.