I agree with John. A bouncing speedo is usually the cable. You can pull the speedo out and then pull the cable out of the housing. Coat the cable with a good grease and push it back into the housing.
As for the speed difference, greasing the cable won't help that. If you have 70 series tires and the originals were 75 series, you have a smaller profile tire with a smaller circumference. I think there is only about a 5% difference, but is the 10 MPH difference at 30 MPH or 105 MPH? You are a braver man than I am to get these old birds up to that speed.
I put one of the aftermarket shrouds on my car, but don't have drive time on it to see if it really helps. Have you tried to install a flex fan? I had one on my car and it really pulled a lot more air, especially at idle, but I couldn't get the shroud to fit with flex fan, so I put the factory 5 blade back on. In all honesty, I wouldn't worry about 212F, as long as you have a good 50/50 mix of antifreeze.
Can't really help with the manifold problem. I had my manifolds decked (surface ground flat) and never had a problem after that.
I didn't look, but do you have any pics of your car posted. Eventually I will end up with a plain Jane Corinthian white car. ;-)