Intakes are available for either two ones or a single four. Tony has done the four with good results with a 500 edelbrock. On my friend's 261, we had dual Rochester ones, and switched to dual twos. We used the Holley-Weber progressives that Langdon's Stovebolt sales. Had to use adapters, he has them too. The Webers made a huge difference, with a full race came he has his idle set to about 650 rpm. With the progessives he still has good low end torque, and has gone to 6,000 rpm on the top.
Another easy mod is ignition. HEI's from the 250 can easily be machined to fit, but they are starting to get hard to find. If your points distributor isn't worn out, you can drop in a Pertronix for under $100. My local parts store can get one on a day's notice. Also a MSD-6 box will help too.
Sixpack is right, but you are looking for simple and economical bolt-ons. You might start planning for long-term items, too. A 261 with a ported and shaved head, GMC pistons and a balance job in addition to the cam, headers and dual carbs can easily put you in the 200 hp range.
