Heads

I have a 69 427 BBC that is running 11:1 compression ratio, cam shaft 287-305 Adv/Duration 2000-5900 RPM Range Lift:.510 Intake/.495 Exhaust, an Edelbrock Performer RPM Air Gap Intake, and a 750 CFM Quick Fuel Q Series Carb. I also have a set of 69 427 bbc matching number heads 3964291 427 69 2.19/1.88 108cc L-78/L-71/L-72/LS-6 Rectangular port 325cc intake

but I have been told these heads are way too big for my engine and I will lose alot of torque and horsepower and I would be better off going with aftermarket heads. Is this true and would I be better off using different heads?  

I would use the heads you have but would have used a slightly bigger cam but the combo you have should work well.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

Yeah the reason I went with that cam is the rpm range and it was in the same range for the most part as the intake and I thought it would also probably run cooler on the street  

Who the hell told you that?!

The factory rectangular port big block heads are still some of the best performers out there. Their only disadvanatge is weight because they're cast iron. A lot of your aftermarket heads are based off the GM rectangular port heads.

Stick with the heads you've got, maybe use a different cam. Install hardened valve seats for unleaded gas and you're good to go.  

A few people told me that about the heads. I was kind of confused because I had always heard those were some of the best heads to ever come out of detroit and are what made the 427 such a popular and powerful motor. I decided to run that cam since it was low-mid range power and I figured that would be best for the street. The tech guy from jegs made a good point he said

"The heads are slightly big but being they are factory heads from the late 60,S or early 70,S I would not change them. They do not flow like a head of 325CC is made today. If it was a current head they would be too big to a bigger degree. Technology has come along way since your heads were made. They do not work as well as the heads made today. In this case that is a benefit to you. A perfect example would be that Black and white TV,s were made at the same time that head was. Do you still watch one? People are thinking that the size of 325 is in term of today’s head. Both are 325 but today’s head will way outflow your head. Again that is an advantage to your particular engine. Also the small tube size on your header will affect the flow on that head. It can not flow air in unless it can get the old out. 2" in tube is about where you should be on tube size. AS far as the cam goes. Are you going to rev past 6000? I think its a good choice unless you are going to try to make 6500-7000 RPM. Remember the bigger the cam the less bottom end power. That is what you said you didn’t want to give up. "  

I think aftermarket aluminum heads are going to be better for me anyway. I'm more about power than numbers. I  I just got lucky I found a block with matching heads and crank. The trick flows seem to be a little nicer than the edelbrocks so I may go with them. How much do you think I should try to get out of my heads I have now?

Heads would be complete minus rocker arms and all original numbers are on them they have not been decked or altered only cleaned and checked.

Casting Numbers

Right Head: 3964291
          GM4T
          F39

Left Head: 3964291
         GM3T
         F49