Daniel,
I believe Hoppers Stoppers do provide conversion kits for Mustangs - and just about everything else! Speak to Ken McLean - he's both knowlegable and helpful. A "one-day-it'll-be-finished" project of mine is a V8 powered FC Holden, which Hoppers have been most helpful in providing advice and modified parts for over recent months.
As for the extractor vs factory manifold question, it stems back to a recent incident where I blew an exhaust gasket out between the head and the extractor flange.
Investigation found that there were actually very few bolts holding the extractors onto the head, and therefore the problem was likely to arise again. The extractors appear to be custom made, not an off the shelf item, so they weren't all that well sorted.
Notwithstanding the lack of bolts, it took two of us six hours to remove the extractors from the car - I still have the cuts and grazes to show for my troubles.
The other problem with the exhaust had been the heat generated and transfered to the cabin, due to the proximity of the pipes to just about everything. The casing of the speedo cable had melted, the dip switch would give you a welt on a bare foot, the steering column used to get hot through transfer from the steering box, the pipes vibrated off the clutch slave cylinder...etc etc. The single pipe exit of the manifold shall hopefully alleviate that problem. I shall also be coating the manifolds and engine pipes in "Jet Hot" or similar as the finish promises significant decrease in engine bay temps.
As the current work is all about safety and self preservation, I thought the minor drop in horsepower by using the cast iron headers was a small price to pay. After all, the car runs a healthy 427 side oiler, and to date, I haven't even been able to get the secondaries to open up, without severe clutch slippage and/or traction loss.
Here's a photo of the car taken at Canberra during Summernats earlier this year.
