here is from a website I came across for you. this what it
said.................

http://www.northcoastmarines.com/bulkhead_fittings.htm

also has supplies for after you drill

KurtG wrote:
I picked up a 70g tank last night. This is an old school tank with hang
on the back filters, skimmer, and under gravel circulation.

I was thinking of converting this to a sump, but how hard is it to
cut/drill a hole in the bottom to put in an overflow. And, where does
one find overflow gadgets anyway?

--Kurt