I assume if you buy the std gaskets they will still need trimming with the work you have had done? Best and easiest option has got to be to get one, sit it on the inlet manifold, stick the bolts in the bolt holes for location and then use the ball pein hammer trick on the inside edges of the ports. The edge of the manifold should cut the gasket or at the very least will mark it out for you. Next best option would be too use gasket material and sit it on the manifold, use the same trick to mark out the bolt holes, stick the bolts in for location and then use the ball pein hammer to mark/cut out the rest.
Last resort would be to just use sealant, maybe if he's using it conjunction with the old (trimmed) gasket that wouldn't be so bad

But hey, thats just my opinion, so don't worry. I'm in no position to argue with your mechanic who does this stuff for a living
