The WhisperFlo is almost certainly too big . . . unless you get at least a 24" sand filter.

Manufacturers play games with horsepower ratings. A 3/4 HP Whisperflo will probably push more water than a 1.5 HP Powerflo, and will push the dirt THROUGH the filter, if you don't mate them properly.

Look at the differencee between these bulkhead fittings:

Hayward SP1023 bulkhead fitting
Hayward SP1023G bulkhead fitting

I think you want the 2nd one. Here's the Hayward manual, and a PoolCenter page that explains more. Depending on shipping costs, the Pool Center may be cheaper, too.

Please let me know which bulkhead you use, and it works.

PS: I'm still working this page, but you may want to look at it and this one, to see what I mean about HP vs flow vs filter capacity