The manifold should work. My stock manifolds (intake and exhaust) from the 305 are now on the 350.
The Quadrajet can be a great carb when it's not worn out and is adjusted properly. A lot of people like them for their "off-roadability" when climbing steep hills or being at odd angles. They apparently work better than most carbs in that aspect. Knowing what I know now, I should've rebuilt (or had someone rebuild) my Quadrajet rather than go aftermarket. I wasn't trying to build a high-performance engine, just a good solid street engine that would get me there and be able to tow when required.
If you're thinking of installing that 350, I'd make sure to test all the usual stuff like compression. Check the oil in the 350 for any weirdness, like particles of metal that would indicate heavy wear of parts, etc... Basically inspect everything you can before removing it.
I'd replace all the gaskets while it was out of the truck. Gaskets aren't terribly expensive but can be a real pain later if they decide to go out on you. And if it doesn't compress well I'd go ahead and do a ring job while it was out and apart. Again, not a huge expense if you can DIY. Make sure you've got a good torque wrench.
Reconnecting all the factory emissions control equipment would help ensure that it still passes inspection.
If you have the space and a little help from a friend (to pull the engine and get it on a stand) you could probably have the engine inspected, removed, torn down, gaskets replaced and back together in a few evenings. Then I'd allot a weekend to get the 305 out and the 350 into the Blazer.
Unfortunately for me, I live in an apartment (gated community) in Midtown Atlanta, GA. So I'm unable to do any real work here at home. Because of that I've lately been wondering if there's any local Chevy truck clubs where people can get together and work on each others vehicles and "talk shop".
Good luck to you... This is a pretty good forum for picking up tips. I've learned a lot by just reading posts and not necessarily asking questions.
~jp