The cast MP valve covers are a pain but once you know the secret to installing them they seal up perfectly. Unlike the stock valve covers you can't just plop them down and put the bolts in. I found this out the hard way one afternoon. What you need to do is this:
1. Attach the gaskets to the valve covers with some ultra RTV. I use the cork gaskets and have had no problems so far. The set on the car has been on it for about 7 years now and I have no leaks.
2. Once attached, apply a thin film of RTV to the gasket so that it will seal to the head. Make sure the head is painfully oil and grease free! I use carb spray and a paper towel. Apply the carb spray to the paper towel and swab the gasket mating surface. Some really long cotton swabs would to the trick as well. You'll more than likely get some oil on the head as you are cleaning. Make sure to clean it up and move on. I usually do the lower surfaces first since they will require the most care.
3. Place the valve cover in place and install the lower 4 bolts first and snug them down but don't tighten them down all the way. Next, install the upper bolts. You'll need to work them into position and they will go in at and angle that seems all wrong. It seems the cast covers are a bit oversized. Tighten the lower bolts down tight and then do the upper ones. This is a must. You MUST do the lower bolts first because if you don't you'll never get the lower bolts into their respective holes. Ask me how I know. It took me about 1.5 hours to figure out this routine one day. What a pain in the butt!
Another problem you will face is that the indentations for the hold down bolts aren't big enough to clear a thick walled socket. You need a thin wall socket if you can find one. The smaller diameter the better. When you are tightening down the bottom bolts it can be a pain to get the socket on and off the bolt head.
This is the routine that works for me. Hope it helps.
