There is an Inliner out there that actually makes an adapter that bolts to the 250 head that allows you to bolt the Stovebolt valve cover directly to it. So its not likely bolted over the top of the 250 valve cover.
The chevytalk post is pretty specific about this aspect of the engine:
"Topping the block is a custom valve cover from Starks that basically uses a stock 216 cover that sits over the 250 valve cover—making the engine look older (and therefore smaller) than it really is."