Here are my thoughts (for free)...
I agree with the other posts that the use of the .308 Saiga would require some fitting since the bullet is longer. IT would be more accurate to compare this to a 7.62x54R - the Dragunov and PKM round - instead of a .308 round.
Let's assume you're going the full route, with a pistol grip conversion to restore the Saiga to its intended form. Let's also assume you're going to do "whatever it takes" to make it work.
Magazine fit.
I am not sure that it would be a good idea to "create" the extra clearance by grinding down the trunion. Therefore, I would consider moving the trigger guard back. Obviously, judicious dremel work on the magazine well area would be necessary for whatever magazine fit you require.
You would most likely need to enlarge the magazine well front-to-back and maybe relocate the trigger guard back. I would weld up the existing holes, then redrill brand new trigger guard holes. If the distance is enough, you could relocate it by removing it, using the two existing rear rivet holes for the front of the trigger guard, and drill two more rivet holes for the rear holes.
That would give you a bit less room in front of the trigger, but not my much. If you had to move it no more than about 1/8 - 3/16" then I doubt you would even notice the difference unless you wore gloves or had real fat fingers!
Bolt & Trunion.
The .308 is over-engineered for the larger round. Of course, so is the AK for the 7.62x39 round. I would have to dig out my ROMAK-3 parts to look more closely, but looking through the plastic, the bolt looks just like a 7.62x39 bolt with an RPK/Yugo style trunion.
Assuming that's true, two conclusions can be drawn - you should have enough meat to accomodate the 8mm round if you convert, and OTOH a $100 Yugo kit could be employed without needing to retrofit a more expensive gun. Point being, that maybe a parts kit conversion would be better?
Ejector.
The same would be true for the ejector. You'd *probably* be OK using a default .308 ejector, but if not you could relocate the Saiga ejector back by grinding the front. You may consider even putting a weld bead on the back just to give it some more meat.
Gas port.
A bigger hole would mean more gas bleed, meaning more kick and a more energetic (read: violent) cycle. The higher-power round may need a smaller gas port to keep the action from cycling so harshly. I'd consider a recoil buffer as well.
Ammo.
8mm ammo is like any other full-power round. You have your ammo and then you have your HOT ammo. HOT ammo as in on the right side of the reloading charts. HOT ammo that is good for a thick machine gun or bolt-action weapon. I'm sure we probably have all read about old chambers failing due to crappy ammo of different calibers. I can think of threads I've read about 7.62x25, .308, .303, and 7.62x54R issues. I'd be real hesitant to shoot just any old ammo through it, since you could have some safety issues there. Reloads would give good control of course, but if you find a good ammo selection I'd buy a bunch and stick to it.
I could almost guarantee that all 8mm ammo is corrosive... dunno about that, but something to consider. A nice SS piston from hotbarrel would be a must-have part IMO.
US parts - the parts count is obviously an issue, and that would be easier with a parts kit conversion, since the Saiga would need a piston (1), FCG (2,3,4), pistol grip (5), and then the 6th part from wherever - muzzle brake? US buttstock? Floorplate?
Other than those issues, I'm at a loss as to what else I would consider for the conversion. Could it be done? Sure - if not then the Yugo M76 would have never been built!