I'm going to keep the 307 in my 85 442. As much as I am tempted to NOT do it, I am bound by the rules of factory equipment. I've got a perfect 455 that would do nicely. But that's not the path I chose.
One thing a totally stock G-body will have over a modified one is that if you do sell it, your potential buyer pool is virtually unlimited. If you buy a rock-stock vehicle that's never been butchered, and you want to butcher it the way you want, then it's a good thing. Flip side- If you want to buy something that's rock stock because that's the way you want it, you can get that too. After you yank the engine and put in an LS or something, your potential sales pool will decrease. You may find that one fish that will pay good money for your mods, and those are great stories to tell, but usually you're not going to get back the value of your time and $$ of modifying it.
But potential sales data isn't why I'm doing it stock. It's not even because it will be an oddity in the sea of non-original G-bodies. I'm going to the way-back machine to maintain the car to its original appearance as close as possible. It's going to cost me much more than it's worth, but I simply do not care. It's an investment for my pleasure, no one else's. There's very few of us doing it. Very few. I know I don't have the economy of scale to do everything exactly the way the factory did it, but I'm going to try to come close.
I still would not hack up a manual transmission car unless it was already hacked. But that's just me. Your mileage may vary.