easy to tell if it has a vacuum leak on the top side .. start it up let it idle and spray carb cleaner around it .. if the idle changes you'll know it has a leak
typically it'll raise the temp some but still usually not enough to make headers glow
Get a timing light on it to see where you are at would be the first place I'd go ... 2nd would be a compression check to see where that is, you'll be able to tell if the cams way off.