[Jeff Lee]

All I know is the late Bob Pott (K/S & L/S 5.0 Mustang); now driven by his daughter Kelly Pott-Hall) switched from a 4-speed Jerico to a 5-Speed. He was told by "all the experts" the cost difference of a 5-speed wouldn't make any noticeable difference, so that's what he ran for all those years.
Then he made the switch. With no other changes he picked up two tenths.
I'm under the impression the less the torque and the less the RPM capability of the engine, the more of an advantage a 5-speed has over a 4-speed.
And if there was a difference of $5K between the two offerings and I thought I could get two tenths without any other changes, I think it is money well spent.

I'd also highly recommend the Sportsman dual 7" clutch assembly. The idea here is to keep the weight as central to the crank centerline as possible. Again, I think small engines really need this. PM me if you need more info.

By the way, I think an '03 MACH 1 would be a really fun stocker! What class does it fit?