the heads are just filled with crap. so were the exhaust manifolds and the intake manifold was cracked and filled too.
-crap...as in cow manure? cabon? JB Weld? Mud? Depending on what it is(was) means it may be removeable, and you should know what it is so taht you don't get it again!

but exhaust i'm not sure. ithink that thats still stock. i have a quadra jet edelbrock carb. cam? and distributer? i'm not sure.
----before you can diagnose what is wrong with your current engine, and therefore decide what to do first, next, etc, you'll have to know what you got! -Edelbrock makes many carbs...Quadrs-Jet is a trademarked name for a Rochester made carb. So you have a spreadbore 4 barrel by edelbrock? What numbers are on it - see Streets for tath, he may be able to identify it, or check the summit or edelbrock sites. Most likely a 650 CFM carb with vac secondaries...but there are other possibilites. It may not be set up right. Did it come on teh 305 or did you move it from teh 350?

Cam...stock or aftermarket, if aftermarket then what cam? The cam is the brains of the engine - it will determine what RPM youwill make power at, and to a lesser degree what you should think about for intake, exh etc.

As to the bad MPG...check the plugs, timing, cap and rotor...does it start and run OK, idle OK, etc? If the exhaust is stock you will have cast iron 'log' style manifolds and a Y pipe and 1 tail pipe with a catalytic ocnverter in it (most likely...what car is this again? what year?)

You pulled the 350 and put in teh 305? Where'd the 305 come from (what car)?

By MPG sucks...what is sucks? 15 is OK, 20 is good. 12 is bad, (for a normally driven camaro with a 305) BUT that depends on other factors - a 4x4 truck with 4.10 gears and a load of engine blocks in teh back might get only 8, so 12 would be GOOD for that vehicle...with a 305. If youare diring it with the secondaries open on the 4bbl, or the powervalve is mis adjusted, or timing is retarded (or vac advnace inop) then you will get bad MPG.

We need MUCH more info than you are giving us!