Based on what I actually am running on my 250 I can say for what you want then use the Offy manifold, the Comp Cams 260H, and the Stovebolt manifolds. I'm using the Clifford Street Rod block hugger style headers but more for the looks.
The Clifford single 4 bbl intake is an excellent mid to top end manifold but doesn't sound what you are looking for. Their dual 4 bbl unit is even further beyond your needs.
I run an Edlebrock 500 carb on the Offy unheated and have no problems summer or winter. Others with the same set up have needed to run heat to the intake so there is no hard and fast rule about it.
The Comp 260H is a great cam for bottom to mid range and besides the grunt it will give it works really well with the intake and exhaust mods you have in mind. You will feel the difference.
Comp recommends using their matching springs but if you look at the specs they are basically a stock replacement. If you are freshening up the head I would recommend just using the new Comp springs instead of shimming and playing with the originals. You can reuse your Chevy pushrods and rockers as long as they are in good shape.
