This is a topic that has been talked about several times here before, you might want to check the archives about previous discussions.

Basically, if you just want to make the engine run, you can drill the steam holes in the 305 heads (use a 400 head gasget as a template) and bolt them on. As I said, the engine will run, but as far as drivability/preformance it will be marginal.

Most of the old timers/geezers here will tell you that a lot of their knowledge was gained by trying things for themselves (especially when they were young and didn't have a lot of money). If you HAVE to get the engine running by all means try the 305 heads. If you have the time, as T/A pointed out you'll be a lot better off at a minimum finding a set better heads (I would say at a minimum a set of 1.94s off a 350) than at some point trying to make the 305s perform.

One thing about "bargin" parts. If you find that the $20 heads you bought give you too much compression to be able to drive the car (or you have to pay the $.20 a gallon difference for premimum fuel to be able to drive it) your $20 for the heads plus the price of gasgets and your time has been pretty much wasted, and you don't really have that much of a bargin.

One last thing on the heads if you decide to use them. The 305 heads (as with most of the latter model heads) were a thin casting and are prone to cracking. If you decide to try these heads, have a machine shop check them for cracks prior to installing them.