Re: '76 ZH Fairlane Marquis
Even just changing your air fliter to a more efficent type like a K&N will make a difference. Also if the engine is being rebuilt, a mild cam such as something in the 200/205 duration at 0.050" lift would give an increase in torque but still good economy and work with stock convertor/diff gears.
With the heads, a good basic rebuild, new guides and teflon seals, hardened exhaust valve seats so you can run unleaded fuel. Also it would be wise, while the heads are being done, to get a 3 angle valve job and blend of the bowl area. This is the best value for money improvement on a set of cleveland heads.
If not already fitted, an electronic distributor and ignition system is a must. If you're still running a points distributor, definitely change it. Either a later model Bosch set up or any reputable aftermarket system will make a big improvement on the starting and efficent running of the engine.
I agree with Dmax, a set of Pacemaker interfernce type extractors and a dual 2.25" exhaust system with a H pipe will also make a noticeable improvement.
Just my 2c, but hope it helps.