The half-moon plastic plugs under the valve cover at the back were at best an engineering fix to fill the gap where a rear mounted distributor would be. It doesn't take a lot of crankcase pressure from a neglected PCV system to blow them out and let engine oil get blown out the back of the engine. If the plastic gets old, brittle and shrinks then by all means it's best to replace them, but other than that it's best to simply put a supporting strap behind them to keep them from blowing out. Threaded holes for the distributor are already present in the back of the block on either side of the plug, so you just need to fashion a simple strap out of any kind of metal with a hole in each end and find a pair of small metric bolts that will thread into the holes. IPD used to sell the strap and bolts, but it's just as easy to make your own.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now