What diameter headers would you suggest for a 383? The current headers on my 350 are 1 5/8" with 3" collectors. Will this be too small of a diameter for the larger displacement engine?

Depends on how built the 383 is. 1 5/8's should be fine for a mild build (like 400hp or less, 1 3/4 for over 400 - 550, 1 7/8 for more.

So being very general, a low horsepower 350 CID V8 engine making only 1.5 horsepower per CID (that's 525 hp) may only require a primary tube size of 1 5/8"stepped to 1 ¾". This particular engine would only require a merged collector throat size of approximately 2 ¼" with a 3" outlet. The primary tube length is dictated by what RPM the engine is run at to obtain these power levels, with the vehicle weight, transmission, torque converter, and type of racing playing a dominating role.

http://performanceweldingheaders.com/primary.htm

Here's a 568 hp 383 that uses basically the same setup...

http://tonybarkerracingengines.com/383ENGINETEST