Not to hijack as this may be relevant to your search for an inexpensive manual transmission option. Have you given much thought to the later modular mustang transmissions?
The T45/TR3650 are on my list of possibilities(providing i keep my car, long story).
The T45 is basically a 5spd T56 and shares some internal parts. They are suppose to be buttery smother shifters. It was the basis for the Keisler RS transmission. The Company theT45source.com has an adapter plate to ditch the integral bellhousing and bolt it to a TKO bellhousing.
mThe trans looks pretty narrow and companies like Hanlon/promotion claim to have versions that will handle 700hp. Stock is rated at 350-360(correct me if i'm wrong) and people have gotten by with more providing they stayed off the slicks.
Then there is the tr3650, its bigger(by how much is unclear) and the adapter plate is a little more involved because you need a 01-04 trans with an 05+ input shaft. Astro performance has one they rate to 1000hp and 700ftlbs. Stock rated at 360tq or 365tq
Upside is that either would bolt to a TKO bellhousing and allow you to use a 130T flywheel. Downside is that its all unknown territory. I don't know the diameter of the input shaft for the pilot bearing, and fitment in the stock tunnel is a guestimate.