Its ok for about 2-3 weeks then its starts to leak again. Often its in the rear part on the pass.side. I will change the gasket this weekend and would appreciate some tips. The heads are noncenterbolt ones.

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I would check to make sure the flanges on the covers are straight. Also grab some of them "load spreading clamps" so the bolt torque isn't concentrated in only 4 spots.

I usually put some silicone or weatherstrip adhesive on the cover and stick the gasket into the cover. This way you could remove/replace the covers without tearing the gasket, and they always stay put in the cover.

I use the fel-pro blue gaskets and must have removed and replaced them 15 times or so in the last 20,000 miles and I never changed the gaskets. Mine are centerbolts though...

Prolly the spots where the bolts go though are "pulled" up from over tightening them, then they always seem to leak. You could hammer them down or... those chrome covers from gmpp are very nice and inexpensive (around $30)

 

I've never seen an old small block that didn't leak around the valve cover eventually. Just a design flaw. That was one of the biggest reasons for the centerbolt redesign. That and GM wanted to keep the aftermarket hopping. Use the load spreaders and try to find a rubber gasket if you can. They work real well.