The 1-3/8" hole will fit a standard 3/4" bulkhead. So you're good to go there.
But are you sure about the size of the other whole? 1-5/8 hole size is typically for a 3/4" sch 80 bulkhead.
But a 1-3/4" whole would fit a standard 1" bulkhead.
I'm taking an educated guess and going to say the tank was already drilled and set up to incorporate 3/4" returns and 1" drains