The Clifford 4 bbl and that 390 is just plain TOO much for a stock slant.Look at the size of the plenum/runners on the intake they are HUGE. Since you already have the 4 bbl,and headers its time to put some money into the head and cam.Port/Polish the head with a valve job(big valves not necessary,but will help) and go with a new erson cam with whatever specs you would like...and bam,you will get a huge increase in performance and economy(if you can keep your foot out of it) If you dont go over 55,I would reccomend some gearing too. some 3.23's or 3.55's would be perfect for what you do.
Also you need to work on your own cars,mechanics will screw you like an all night glory hole if you give them half the chance.
