Simple Answer:-
If you already have a Holley 500, and have your heart set on it, all you need to do is reduce the size of the power valve channel restriction to around 21 thou for each hole in the carby body, and start with jets around the 65 in a car with a stock cam and a set of headers. If you have a 350 Holley, you could drop down to 56 main jets. Any time you use a Holley #2300 carb on a 200, I'd be going straight to tubing headers. There is no advantage in using a bigger carb like a Holley unless the engine is able to pull 125 hp with a Holley Weber carb.
Get a good Holley book, and talk with the Ford Six performance people at
http://falconperformance.sundog.net
Why?
It's all to do with air flow. Big jugs need lots or air flow, little carbs don't need so much. A worked engine can consum a lot of air, and a bigger carb can sustain a higher air speed through the carby.
In applications where there a little 2 liter Pinto engine kicks out 138bhp at 6500 rpm, a 500 cfm 2-bbl #2300 works fine, better than a 350 cfm in fact. There is a loss in fuel ecconomy when you over carb an engine. For intance, at the 130-140 hp level, the main jet size goes up from someting like 65 to 74 when you change go up in carby size from a 350 Holley to a 500 Holley on a four cylinder car. You could loose quite a few mpg if the carb is too big.
On a six with a good set tubing headers, the stock 85 net hp at about 3500 rpm will go up to 100 hp at 3800 rpm if the carb is big enough. The main limiters to a getting near the 138 hp that a good 500 holley could use are the size of the hole in the head, where the original 1-bbl carb sits. If its 1.5" in diameter, even a 350 carb is a little to big for the area of the holel. If its 1.75" in diameter, like most post 1969 heads, then a 350 cfm carb could be good enough.
Generally, in a stock application, a 280 Holley-Weber (Pinto carb) is a perfect thing. I'd stick with either of these, and then go for a 350 Holley or 500 cfm if or when you've done a change to a higher lift cam and are prepared to run that 1.75" hole log head.
