Im with chad i know you want to keep up with traffic on the insterstate but i still wouldnt be going that fast in the car especially since its a big journey take it slow and give it time i would stick to 65 or slower preferrably 60 thats kinda a sweet spot for these cars they like to drive at that speed for extended periods of time. Did it for well over 4 hours 1 day in a round trip no problems.
Now i remember you saying the car runs like a pile of poop so i would get that figured out first whether it be fuel compression or spark it has to be 1 of the three ( could be 2) once its running better then move onto the charging system making sure it wont die on your while your driving, next the cooling system does the radiator leak does it need a new thermostat? is the water pump effectively moving coolant does it over heat? are the tubes in the radiator clogged? all things worth checking out, then go to the transmission and drive train make sure it shifts properly, does the clutch move ok? does it fully disengage? make sure theres fluid in both the trans and the diff, make sure your axle seals arnt leaking on the brakes, are the bearings good in the trans and diff? btw i would test the drive train with the rear jacked up so you dont actually move to allow the moving parts to loosen up without much stress, lastly the brakes now this can go in front of the drive train but i feel having a nice braking system doesnt do you much good if you cant even go anywhere, check all the wheel cylinders they likely need replaced as would the hoses, make sure there are no kinks or holes in the brake line check the shoes and drums make sure they look ok and dont have excessive wear also look for any build up of grease or fluid as this could be a sign of leaking cylinders or axle seals, beware that the line nut may be rusted to the wheel cylinder meaning you might have to replace the whole like or attempt to splice in a section, more than likely you need to replace the tires, (very good idea) i tried driving on the ones that were on my car when i bought it from the late 90s so only about 10 years old at the time and i had one blow on me while i was driving it, now you can drive it to check the suspension and steering, i like to turn the car back and forth within the lane to get a kinda sway, it loads and unloads the suspension on each side which helps with trying to find clunks from ball joints and bearing noise from bad wheel bearings, suggested to repack the bearings if you are gonna use the originals or for about 40 bucks or less you can replace the bearing races and seals for the front, also feel the steering is it super loose or clunky? some of it could be play in the box some could be from bad tie rods. after all that feels good then and only then would i feel safe driving it any sort of distance.
When you see the guys from roadkill do those crazy trips they usually check most of that stuff as they are working on the car to make sure it isn't about to fall off or attempt to kill them as they are driving but they also have backup in case something goes wrong, something that you wont have so be overly cautious about it