Hi Peter,
I went thru 2 Saganaw 4 speeds in a couple of years. They both broke while shifting fast. The shifter was never very positive with all this links and levers involved. It was fine for daily driving and did just fit between the floor board and cross-member. I am happy with the T5. It is a non-world class Camaro version. I use 2nd thru 4th up to about 40 and 5th after that, although the engine will pull smoothly in 5th as low as 20 mph. I use 1st for drag racing and starting on steep hills at traffic lights. My 1st is 3.0. I tried an S10 T5 but the first gear was way too low. It has lasted many years and still shifts great. But you have to cut the middle section out of the cross member to get the T5 under the floor board. This required a V shaped piece of angle iron go behind the T5 and bridge the gap in the cross member. This also has to be bolted in and removable to change the trans or clutch. Without this reinforcement the frame will bend in the middle.

I switched to the Mustang II front end because the the car did not steer or stop well - even with a vacuum power attached.
You are correct that most mustang front end kits drop the height of the front end by 2 to 3 inches. I welded spacers under mine to keep the car at the same height. If you get one, order the lightest gauge coil springs and the biggest disks you can get.

I kept the old master cylinder because I wanted to save the peddles and clutch. The output line runs along across the cross -member to under the passenger side of the engine compartment where a vacuum booster increases the pressure . It works very well - feels just like normal power brakes. I think you can get disks for the original front end from Buffalo Ent. and I think the Vega steering box can be used to get better steering controll.

With all this said, I often wonder if I would have been better off just getting a modern frame ,like a Chevy Blaser, and just fit the 40 Chevy body on to the modern frame.

As for the exhaust, I have experinented with many. The best setup I have found for performance so far is the dual headers with small pipes. I use 1.5 dia on the single ports and 1.625 on the siamese. These smaller pipes work better because the keep the exhaust pulses moving faster and create a mini vacuum at the cylinder during the cam overlap, causing new gas and air to get pulled in as the intake starts to open. You can feel the boost. Larger pipes do not do this. One other tip is to go to a junk yard and get the entire exhaust system from a old Jaguar XJ6. Its all stainless, the right size pipe and will sound great with your GMC.

Frank

FranK Hainey