Bulkhead size

[clancaster23]

I want to put new bulkheads in a tank I picked up before I get it on the stand and running. It has two overflow holes in the center which are 3" and also has four 1 1/2" holes for an internal loop flow system. From what I've read, I should get 2" bulkheads for the two overflow holes but I'm not sure on the smaller 1 1/2" holes or even if the 2" size for the overflow holes is right. Also, is threaded or slip better?

 

[emmett0122]

SizeHole Dia.
1/2"1-1/8"
3/4"
1"1-7/16"
1-3/4"
1-1/2"2-3/8"
2"3"

so yes the overflow would be 2" and the returns would be 3/4"

 

[emmett0122]

size _GPM __GPH
1/2 = 2.50 = 150
3/4 = 5.63 = 337.5
1.0 = 10.0 = 600
1.25=15.6 = 937.5
1.5 = 22.5 = 1350
2.0 = 40.0 = 2400