68 c-10 250-6 overheating

[dwmurry2006]

Heres the problem. I got a 68 c-10 2wd longbed with the 250 6 cylinder and 3 speed saginaw. Factory 2 core rad. and the old 4 blade fan. Just within the past month it started overheating on the highway. The dummy light would come on. I had a 195* t-stat at the time.

Heres what i've done so far...
Drain and flush the cooling system and put fresh coolant 50/50 mix in it.
threw a new thermostat in there to please myself. was told to put 160* in it Overheating really never quit. Checked to make sure water was flowing well- a ok.
Bought a temp gauge to satisfy myself. I could start it up and idle it and it never would get over 185-190. Take it for a spin and pretty quick it made it to 240 while driving. So i pulled off to a exit and it climbed to 280 and sat there. Idled it up and brought it down to 240. Drove it home and never went above 240 but never any lower.

So I moved on... Then found out vac adv. quit on distributor. Was told that it would cause my overheating not having enough timing advance while driving.

found skip white performance had a hei dist for chevy 6 cyl. to upgrade from points.
called and talked to tech support and they said it would drop in and run just fine in my truck.
so i went ahead and bought it. 85 bucks shipped to my house.

drop it in and set timing at 10* btdc like tech told me to do. and plugged vac adv. in fired right up and purred like a kitten.sounded great idled so much smoother. but when you crack the throttle it would start bucking and stumbling then let out a huge backfire, not cool at 2 am when everyone around is sleeping. tried to drive it and its impossible. bucking and stumbling and the backfires...oh my lands.
next morning i call and tech told me it was from my old rochesters port vacuum applies full vac when you barely crack throttle instead of supplying gradual vacuum with throttle. told me to just leave vac adv. off and advance timing a few more degrees.

well dont i need the vac adv for mpg and drivability?

told him i wanted it to work with the vac adv. or i didnt want the dist. since other tech told me it would work just fine. he then told me how to use the allen wrench to adjust the amount of vac adv. well that didnt help at all. so i disconnect and plug vacuum line and advance the timing some more and well.... the truck has a miss at idle now.

IT STILL OVERHEATS. It wont if you let it sit there and idle but when you start driving it goes up to 240 it will go up to 280 when you stop then you take off and back to 240 and stays there.

is this from not having enough timing advance? from the lack of vac adv. or does my problem lie elsewhere. My overheating problem started when the vacuum advance died on my points distributor.

sorry for the long post just wanted to be thorough. please help this is my daily driver.