Here's what I'm thinking,
'67 and newer heads have a different vavle angle and bigger valves, so you'd
need to get pistons with the right valve reliefs.
So , depending on what your block is bored to right now, you could bore it
to make it a 428 and use 428 pistons with the correct valve reliefs for the
6x heads.
If that 421 you have was already rebuilt, it might already be 30 over.
I believe it will be easier to find the 428 pistons than the 421.
As for those 6.6 heads, look and see which one they are. Hopefully they're
6x-4

I've asked a Pontiac head specialist near me what he recommends.
I'll let you know what he comes up with.

Jay S

OK, here's the scoop from the expert.
He says the 6x-4 heads would be a great choice. You may need to mill the
heads to increase compression depending on what pistons are used.
Like I figured, your best choice is a set of 428 pistons with the proper
valve reliefs for the later heads.
I inquired about using your 326 parts and he says it wouldn't be worth it.
It would cost a lot to work those heads and you'd still have less potential
power than the 6X-4 heads.

If you need head work done, it would be worth sending the heads to this guy.
He's been featured in several car mags, High Performance Pontiac, Pontiac
Enthuisiast, Hot Rod.
He's located on the west coast of Canada, but says he ships most of his
stuff to the eastern U.S. He'll ship anywhere in the world.

Check him out:
http://www.sdperformance.com/index.php

Jay S