Should I get heads with 64 cc combustions chambers and stock valve sizes? Or should I get the stock 76cc and the 2.02" / 1.60" valves?

Keep in mind that I don't plan on revving past 5000-5500 rpm.

 

what kind of heads are you looking at? really you can get both out of most good aftermarket heads.
if i had to chose i would go for the smaller combustion chamber. a little more compression is always good for any combo. the 1.96/1.50 stock sized valves are capable of supporting the RPm range you are looking for as long as your cam and intake are correct. the cam and intake actually have more of an effect on the air inlet(they control total volume and peak velocity) than valve size will.
i don't kow what your price range is, but for under $1000 there are a lot of sets of heads on the market that can give you a 64cc chamber and bigger valves, Trick Flows come to mind

later
tim

 

I'm looking at the World Castings S/R heads.

They flow exactly the same as the S/R torquers but they don't make them with 2.02"/1.60" valves. I read all the pdf's on their website about cfm rating and all. Looks like the only time the big valve flow more is above 3000 rpm and .600" lift. No joke.

The set I'm lookin at are 64cc combustion chambers with stock valve sizes and 170cc intake runners. I think these would be good for my combo.

When I install these I also plan on putting in either the LT4 or LT4 HOT cam. I still haven't figured out which one will match better to my intake and stall.

Like I said, I don't plan on running higher than 5000-5500 rpm. And I will NEED some low/mid range power.

 

even if you decide to do the LT4 hot cam, i think the LT4 heads that it was designed for only have 2.00/1.56 valves anyway.
like i said the intake and cam hgave more to do with flow than the valves.
go for the 64cc heads.

later
tim

 

Here's some proof for what Tim just said about the hot cam.
Crate 350 with vortec heads, single plane intake and hot cam made 422hp. ZZ4 bottome end with fastburn aluminum heads, single plane intake and hot cam made 430hp. Fastburns have better everything including larger valves yet the hp difference is only 8. Go figure. I must also admit that the bottom ends are different (crank, rods and pistons) but they're both 350 cubes with the same length connecting rods!
Big valves = big performance only when used with a big wild cam.
My opinion is obviously for getting a 350 instead of heads on a 305. Tell me one person that has been upset with investing in a 350, yeah, no hands .
The only reason I say 350 is good is because I feel very strongly about the no replacement for displacement. I just want everyone to go fast and beatup on mustangs and imports!
Let me tell you, when you figure out the hp per dollar there are 2 things on my list. N2o is obviously the best but then comes an engine swap.