Need help with bulkhead sizes

[rocketlily]

My new tank has a 2 1/2" hole for the drain and 1 3/4" holes for the returns. What size of bulkheads would I buy for this. I don't seem to find 2 1/2" and 1 3/4" listed on retail sites. Also it it better to use slip in fittings or thread in fittings.

Any help with this and the size of lines to run, to and from the sump would be appreciated. Size of tank is 97 gal. and sump will be approx 30 gal.