340 exhaust manifold questions

I was out cruising in the Scamp yesterday and while stopped at a light, the guy beside me asked if was interested in any A body parts. Always wanting to know what's out there I asked him what he had.... turns out he's got some early ('68 to '70) 340 exhaust manifolds (not cracked) that he's willing to sell. I know that I'll need to check the casting numbers before pulling the trigger on them but am wondering if it's worth going the next step based on my engine and plans. The guy is asking $275 for the manifolds.

I've got a mild 318 (1974) that will be getting a new exhaust system shortly so this would be an ideal time to change manifolds if the price is right (not just for the manifolds but also the extra work/costs required for the exhaust system).

If I decide to buy the manifolds, what are all the issues I might run into into when doing the install?

Would it be worth spending ~$275 for the manifolds given the potential extra work required fitting them on my block, etc for the minimal power gains I'd get?

Check the casting numbers, the 68-70 last three numbers are 553 for the drivers and 549 for the passenger side. 275.00 is a fair price.