Your car is probably dribbling the the tires down the freeway due to some bad shocks. I would check the shocks. And tires for cupping that would be a sign of bad shock.
Just make sure you got the right insurance And get the car apprised by a classic car association If the car means that much to you! Cause if some one hits you. its going to be hell.
It must be the headers you are running that are making it a pain. On mine I got a 3 x pipe on with cut out right after the headers and fits nicely. But It could be because I changed the transmission mount to the round type. If no other way summit has Intermediate Pipes for your problem.
Before going and spending a ton of money. check your pinion bearings. If the chrome has come off the bearings. It can give you a nasty vibration with out the noise. check both races and were the bearing rides on. clean well with brake cleaner so its more visible.
I would think the paint would keep it cooler! Like When a painted bottom on a Intake. Hot oil will drip off and not seat on the casting forever. Plus air pushes of paint more Easily than a Rough surface. That's my Theory.
What kind of inline filter? And what is the air fuel ratio read at high revs? I just realized your in Australia. Your altitude might have some thing to do with the problem maybe even the temperature it can be a killer in power!
If your close to stock I would stick to the Edelbrock. Thay work well with proper tuning just like the thermoquad carb. I personally like them better than holleys and proform vacuum secondary. Dp work awsome but can bog the car out if not on the right set up!
A transmission on a car should be capable of pulling 2000 pounds on flat land. on steep hills about 1000.
But it all depends on how many miles that trans has. A good cooler with fan,and trans temp gauge would be a plus. Also make sure your tire pressure is always good. you don't want it fishing...