Thanks everyone for the support.
Barry: I'll have to check the exact spring pressures. The cam is a street roller with 0.02 lash, so it nets 0.620 lift. I was running lighter springs in the past. I too was worried about valve float, so I changed the springs to a higher seat and open pressure. At this time I don't think I am getting valve float. I used to run the engine with a Edelbrock Victor and 850 Mighty Demon. The engine chassis dyno had max hp at 5900 with the lighter springs. The flywheel hp is calculated based on a 18% drivetrain loss. The T56 and 9" Ford Locker should be close to this number. The car weights 3400# and ran 7.1 sec and 125 mph in the eight with the four barrel through a C6. I also ran the exact same time with 4 Weber IDA 48's, This equates to about 520 to 550 hp at the flywheel. The intake runners are about 12" to the head, or about 17" total. This puts 4rd wave tuning about 4200 rpm and 3rd wave about 5400 rpm.
Jay: You are exactly correct. The drive-ability would improve with a progressive throttle. I may fabricate a better linkage in the future, but presently she is manageable. My main constraint of larger throttle bodies is the cost. TVM's cost about $4000. "Speed costs money, how fast do you want to go?" I think the intake is restricting the upper end and my first effort will be to port it. I have a tig welder and can weld aluminum. If I can't get enough airflow I will switch back to the Victor. Modifying the Victor intake for BT heads would not be a problem, but I sure like the idle and low end torque of the IR setup.
I previously used to have an Edelbrock Victor on the car, and bracket raced it occasionally. After a few years,I plumbed the Victor for a fogger. The car has dual fuel tanks, dual fuel pumps, dual feed and regulators. The first time I used the fogger, 100 shot, the engine blew through the 4500 rpm converter. I replaced the converter with a tighter one, 3000 rpm. Then the 2nd gear band began to slip (C6). I was working out the kinks; but unfortunately, the local track in Holt, FL stopped paying their taxes, and the county shut them down. I decided to convert the 68 back to the street. I removed the shock towers, dropped the engine mount down 2 inches, and replaced the C6 with a t56. I also converted her from a single carburetor Victor intake to the Megasquirt EFI 8 stack induction. She is now very street-able.
Although I don't race on the street, I did hear of someone in a 68 Mustang fastback embarrassing a late model Dodge 392 Challenger.
I am a hobbyist, I have been fabricating and modifying this mustang for over 10 years. I like to think of these cars as works of art. Not just for their styling, but as extensions of our mechanical skills and ingenuity. These cars represent the best of hotrodding. I have built several turbo setups, and could easily push the horsepower over 1000 with forced induction, but I would like to think we value the classic appearance and aesthetics of this generation Ford.
When I upgrade this engine, it will be with this philosophy in mind. It will be better heads, better intake, better cam, better execution of the classic design.
I digress, but I think the next step is better heads.
Thanks for the help.