1-5/8 versus 1- 3/4 headers

Hi guys, anyone seen any comparisons between 1-5/8" and 1-3/4" headers? I have a hand made set of 1-3/4" headers on my 360 in a right hand drive Dart in Australia, and I'm trying to get my ' 69 Barracuda going. The only headers available are 1-5/8" Hookers Hedman etc without going to the mega buck TTI's.
I'm wondering if there is any performance loss in going to the smaller headers. Anyone got any thoughts or info?