That's the right idea. It's kind of creepy that the seller didn't even clean that manifold before fogging it with the wrong blue paint. It's also not a 1974 manifold. I can't see the date code next to the water neck but it looks like an earlier '67-'72 manifold which is better since it doesn't have EGR. The carburetor number given in the questions looks like a 1971 Pontiac with an automatic.

You might be able to do better if you research it a little and look for the parts locally. But if you want to take the risk, those look like the parts you're looking for.

Next, you'll need a throttle cable and bracket. If you have an automatic, you'll need a means of controlling your downshift.

It would be a good idea to upgrade your cam when you make the swap. Something like this:

http://store.summitracing.com/partdetail.asp?part=SUM%2DK2801&autoview=sku

Here's a nice air cleaner:

http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM%2D239443&N=700+4294756384+400304+4294904151+115&autoview=sku