All versions of the 6X head are D-port.
Round port heads are quite rare. They only came on special Ram Air, High Output or Super Duty engines from 1968 - 1974. Probably 99.999% of all Pontiac heads are D-port.
Thread showing the differences between round and D-port heads:
http://forums.maxperformanceinc.com/...d.php?t=636309
Finding headers for your car should be easy, they’re available from several different manufacturers.
With that said most will be dragging the ground and become a headache, some Ram Air exhaust manifolds might be a better option if you’re not building an all out race engine.