Well, after spending a few days searching for suitable donors and trying to decipher all of the necessary information on what donor parts are actually required, what donor years give you the parts you need in the right configuration, etc., I came to the realization today that I still know very little.

What I do know is that it seems that I should no longer keep looking for a donor. Since I don't need the engine and transmission, a donor won't give me much in the way of value, and the extra parts won't really have enough value to make it worth it.

Here's some of the stuff that I learned about donors for FFR kits, I thought I'd put it here for archiving and if someone else is looking.

What years started disc brakes on the rear? - 1994

Can I use a V6? Yes, but it'll have a weaker 7.5" rear end, not advisable

Can I use a 4cyl? No.

Any reason not to use a 94+ donor? Yes, the axle width is wider, which will mean more limited wheel options (backspace) or you will need to change the axles.

Will the stock rear end have the right gearing? For most applications, no, the stock 3.08's and less gearing will want to be replaced to 3.55's or 3.73's at least. This will help the relatively mild stock motor get a little more punch and put more power to the road, so to speak. As horsepower increases through add-ons and upgrades, wheel spin could be come an issue with the 3.55's, etc., and it will likely prove useful to go back to using 3.27's or 3.08's to ensure the rubber continues to meet the road effectively.

What Year is the best of all worlds? 87-93 5.0 Mustangs, they have the 8.8" rear end

I'm going to start researching individual parts now, and see what I can come up with.

Some 302/5.0L Stock Engine Build Up Info I found (thanks to Oldtymeflyr):

Go with a 85 Mustang Duraspark distributor, rebuilt it might go $50.

Recently there was a discussion comparing intakes, the Edelbrock Performer worked better than the RPM manifold.

Cams, with a E303 cam, an early Performer type intake and with a set of stock GT40P heads, I am right at 300 RWT. Also, I run a vacuum Holley 600 and 4 into 4 exhaust.

My thoughts are do a little work on a cheap set of GT40P heads, pick your cam right and you should be a little over 300 RWHP.

Some More Donor Info:

From 66fstbk:

I wouldn't do a "single" donor. Just get the parts you want from a couple different vehicles. You can do it very affordably if you shop at the right yards.
98' T-Bird IRS Complete with posi and discs $250 (requires IRS FFR option)
95" spindles and brakes in excellent condition $100
04' fuel tank with plastic shiled and filler tube like new $100
95' 5.0 engine and trans 74k mi. $1,000 (only needed shortblock and trans, did not need rebuild)

If I had purchased a 87-93 it would likely have needed an engine rebuild, would not have used front hubs or brakes, rear end (because I used IRS). So there was no point of a single donor for me.

True the kits were origionally designed for Fox single donors but a lot better parts from different year cars plus the age of these donors make this a less and less optimal solution every with passing year.

From Sergio:

I've used a 1991 donor to build a FFR MKI and a 2003 SVT Mustang Cobra to build a FFR MKIII .

I think I am qualified to say that the Best Donor for these cars is The Terminator (also Known as the 2003/2004 SVT Cobras.)

87-93 donors come with stock 225HP while the 03/04 cobras make 420HP stock.

While you couldn't probably use a 03 Cobra as a donor for a MKI (unless you do tons of modifications), you can use any 87-2004 Mustang V8 to build your MKIII, except that (thanks to me), Factory Five modified their literature to read: You can use any V8 Mustang from 87 to 2004 (except 03/04 Cobras).

The 03/04 Cobras can be used as donors as I have proven, with some sensible modifications, a handful of others have done it too now.