Cool - sounds powerful! What bolt & trunion are you going to use?
That much gas pressure would mean that there is no need for a huge hole - a small hole should allow enough pressure and not cause a problem with the ballistics.
I would say a small port would be fine - mimic a FAL port or something like that. I wonder what is a factory Saiga 308 port size?
On my 308 conversion, I used a port that is just about 11/64", which is the factory size for 7.62x39. I think it is too big of a port for 308. I am going to add those purge holes around the tube/block connection like on an AK-74 to help purge some of the gas.