If it don't make power til 4500 rpm, sounds like your cam may be too big.

So what are your cam specs ?

If having too much cam is your problem, maybe a smaller cam will bring your power band back down where you want it.

Another thing that can reduce low end power is having a slow timing curve, & one that doesn't begin to come in at low rpm. Most seem to agree that the advance springs & weights should be adjusted to where they begin to advance the timing just above 1000rpm, & apply full advance at just below 3000 rpm.

Many HEI's have stiff advance springs that don't start advancing at a low enuff rpm, & don't reach full advance 'til after 4000 rpm. You can probably see how that could put your power band up closer to that 4500 rpm you mentioned.

So, have you ever checked your timing curve ? Do you know what it is, now ?

I'd also recommend a GOOD, properly rebuilt & functioning Q-jet carb, rather than that 650 Holley. You mentioned an iron intake. What type adapter are you running under that Holley, to mate it to the spread bore iron intake ?

The Performer will make power mostly from idle to 4500 or so.

The RPM is more for mid-range & higher rpm.

The factory iron intake is supposed be the best of the 3, all-around.

If I understand your post, you are asking about a set of alum heads, intake, & possibly a roller cam. Is that correct ?

My advice is to determine exactly what you have now. A new timing curve and/or a different cam MAY provide the power you seek, and save you lots of money.

MOST guys don't recommend putting new top end parts on a well used bottom end.

But, if you are gonna do it anyhow, I'd go with the smallest cfm D-port alum heads. You don't need the big cfm flow. It will reduce low rpm torque & power.

Also, which ever type cam you decide to go with, stay on the smaller, or more conservative side. It don't take a big cam to make 1500-5500rpm power in a Pontiac 400. If your car will be mostly street driven, you want more low rpm torque & mid-range power, rather than power above 5000 rpm, where you don't go very often.

If you don't wanna bolt an iron intake up to the alum heads, you could buy a factory alum Q-jet intake, like used on the RAIV & 455HO engines. There have been aftermarket copies of these on the market for a few years. If you decide to do this, be advised that some of these intakes have been found to have ports that didn't match up well with the intake ports in the heads. Some needed some port matching work, before use.

https://www.classicindustries.com/product/14420.html

Parts of these videos are rather slow moving & longer than they needed to be. But they show a lot of detail about the reproduction intake.

https://www.youtube.com/watch?v=lrQFB-hNKqo

https://www.youtube.com/watch?v=TAR89F7IXvk

https://www.youtube.com/watch?v=aVmQGOzdphs