I agree with porterdog's suggestion. I have a '77 930 with the original 'non-Turbo" exhaust valve covers. I have NO oil leaks
. I am using the red silicone covers now- installed when I last tuned it, but had the normal graphite ones before with no leak issues. IMHO, the Turbo valve covers are nice, but not necessary. They do need to be checked for flatness- it can be corrected using a large flat file at at angle if that is all you have, or as porterdog suggests, the wet/dry paper on glass- maybe 150 grit to start, finishing with 320. The problem I see with using the wet/dry paper, though totally manageable obviously, is that the sheets of wet/dry aren't long enough as the cover is stroked back/forth...... maybe some industrial supply shop has longer ones?.......
The quickest and easiest method is to lightly hold them on a belt sander with a bed large enough to do the whole cover at once. Sand for a few seconds, then check how it's going. I did mine in less than a minute each. Be careful to firmly hold on to the covers, as that sander will want to take them out of your hands and send them flying. I have not had any drip issues. I live in the mountains at 8000ft- so it gets pretty cold here as well, and the car sat for a few years ( kids, remodel the house, kids, move, etc...) while I did some restoration work- still no leaks....
You could always take them to an auto machine shop where they could deck them on their head surfacing machine- that might cost a few $$ because they would have to fit it or make a jig to hold something so small.... No belt sander access? Go with porterdog's method.......
Good luck- it's very easy to do......
Tim