"Is assembly with screws really a safe method for attaching the trunnions to the AK receiver."

yes. no safety is compromised.

"Seems to me that the screws would over time, start to work on the edges of the receiver holes elongating them. Can a person really obtain enough pressure with screws to keep this from occurring?"

yes, if properly sized and installed the screws will grip as tightly as soft rivets.

"I believe you have used this assembly method in the past. Have you seen the trunnions loosening with time?"

the only trunnions i have ever seen loosen were a pair of riveted b-west ak's. in those 2 cases it was the rear trunnions that were hammered loose.

"If it's as simple as drilling out the original rivets, removing the remains of the original receiver. Tapping the rivet holes, matching the forward and rear trunnions to the new receiver using the remains of the old receiver as a template for positioning the holes. And then screwing the trunnions to the new receiver, I may go this route."

that's correct. i have shot screwed together full auto ak's that were built by amatuers.

"This would not effect my headspacing correct?"

as long as the bbl. remains pinned to the trunnion, the headspace cannot move. the receiver has nothing to do with headspace.

"I'm an aircraft mechanic and have the experience and tooling to do the work, my main concern is safety of the shooter with the trunnions working loose over time."

again, the bbl./trunnion could, conceptually, be fired safely minus a receiver. the receiver's only structural function is to hold the pieces together and provide an anchor point for the recoil stop (the rear trunnion).