80 MPH with a 235 is wishful thinking, and if you do manage to get it up to that speed you will be a hazard to everyone on the road, including yourself. Dual carbs and headers on a 235 are just eye candy- - - -the truck will usually run better on the single carb/single exhaust. The cylinder head in between those parts is incapable from benefitting from the increased carburetion and exhaust volume. If you want an inline six rig that will run those kinds of speeds, you'll need to go with a Chevy 292, a Ford 300, or maybe a Jag or Toyota Supra powertrain. While you're at it, graft in the front and rear suspension and brakes from a sports car.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!