My experience has been that the size measured is the ID. 1 5/8" is not 1 3/4" OD a the tubing is smaller than 1/16. This is not always true and there are differences between steel and SS, etc.
Superhcharged engines (like mine) need 1 3/4" headers and if they are the Belangers 5>2>1 the pickles must be removed for the headers to make power.
Normally aspirated can use 1 5/8" OR 1 3/4" headers. Highly modified n/a engines also need the 1 3/4" headers for a slightly better performance (gas mileage and/or hp).
1 5/8" headers are best for stock. 'Tuned' (read expensive) headers are the best for all applications, but the difference in hp is often not worthy of the much higher cost.
Ted