has anybody done the oversized piston updrade from Motorsport Auto? I am still thinking about improving performance without a significant cost and the pistons sounded intriguing. What kind of performance boost could i expect out of the .04 oversized? More HP or what?

It's hard to say how much HP you would gain by going to 40 over pistons only. You would increase your displacement a bit, but the total gain would depend on what other modifications you make. Carbs, porting, which head, which size valves in the head, cam, final compression ratio, so to be honest it would be hard to say exactly how much of a gain you could expect. Yes, you'll gain a few HP if everything else is left completely stock, but with a few more mods you could be looking at anywhere from 30-60 HP gain. With just the pistons maybe 10-15 HP, but that's just a guess. I might be guessing a little high. Just remember HP costs money, at some point you can spend a lot of money to make very small increases. If you have a stock L-24 your best bet might be to go to an L-28 (280) bottom end and get a bigger increase in displacement and go from there. There is an old saying that goes "there is no replacement for displacement". It holds true for L series engines as well as a Chevy small block or whatever.

If they are just .04 oversized pistons then they are for when you rebuild an engine and the cylinders need to be overbored to remove scratches, etc. This means new rings would need to be used as well. You would not see a substantial gain in power other than what any refresh of the engine would bring.

That's my thoughts anyway!