need help w/ my 55g w/overflow diy plumming

[yamaha racer]

well as most of u know i got a new tank and me and my dad are setting it up. but there are 2 holes; 1- 1 3/4" and 1 1 1/2" and my fist question is which on is for in and which on is out? today we went to a plumbing place and checked out bulkheads and the inside diamiter for the 1 3/4" is 1" is that how it usually is and the 1 1/2" is 3/4". is that how they are?

thanks

 

[rsteagall]

I believe the smaller hole should be used for the return. You want the outlet to have a greater potential of flow than your return (unless they are the same). If one must be restricted... it should be the return.