I like to use tubing 1/4 inch bigger than exhaust valve and go as straight as I can wherever I can( not lots of tight turns and twists close to the head) this will usually give you an pipe ID about 1/8 inch bigger than the exhaust valve.

I also make an effort to pair the cylinders in 2 different lenghts to broaden the power curve and join the collector in the firing order rotation

Use a merge collector and tapered connectors where you can