can you all please help me, ive been working on my own truck for the 15 years iv'e owned it and im really at my witts end with a timing issue (thats what i belive it to be) It has been running rough and lacking power for some time, and when i have a day to play with it (im always working and this is my p30 stepvan work truck) i have tryed to time it correctly. I have rebuilt the top end a year ago, new heads, intake, holly aftermarket carb, new exuast manifolds, have heat sheild boots on my plug wires because they were always getting brittle from the heat and cracking, i don't have that issue anymore. I have put #1 at TDC and tryed to get it 10 degrees advanced. i have also tryed driving down the road playing with the distributor to see how it affects power. I have an aftermarket HEI distributor. for the last few months its been runing very hot on the highway 210-260 and when i pull over and check radiator is bubling into the overflow bottle. I ran this truck un florida before moving to maine, and always had to run it with no thermostat to keep it cool enough. so when i came to maine i put it back in, was ok for a wile, then when it started runing hot i took it out so it could free flow, didn't help any. Flushed everything and had very good flow put thermostat back in and now just going down the road im at 220 cruising at about 45-50 mph. Now its getting very hot and very sluggish up hills. tryed also adjusting carb mixture screws to see if it would run a little better, no luck. Did i mention that thes 350 has about 600,000 miles on it.........I LOVE CHEVY...lol. So how can i corectly time it? how should i know where to set the carb mixture screws? can all this lead to the excessive heat? checked all the plugs they look good, new about a month ago. Please, any help would be appriciated, seeing how i have tryed a million things and maybe im doing them wrong or out of sequense. i also adjusted my valves the other day thinking that was the cause because i read that if the exuast valve didn't open at the right time or enough the exuast gas would burn hotter. PLEASE HELP...lol thanks to everyone on here for the help you give and opinions or answers you provide to all of us.