Bulkhead Plumbing question

[trix25]

i am just in the middle of setting up my first saltwater tank and kinda stumped at my overflow plumbing.
I want it to go from my CPR overflow with 3/4" bulkhead fitting to an union and to my 3/4" PVC tube pipes. however, the problem i am having is that the 3/4" threaded bulkhead is not fitting any union i can find at my local Home depot. it seems the bulkhead is not a standard size i can get. if i am correct i need a reducer from the bulkhead to the 3/4 union but the 1" reducer to 3/4" didn't fit the threads on the bulkhead...is this confusing anyone? so my question now is what are my plumbing options??? and what is the size of the bulkhead threads?

here is a link of my bulkhead


edit: sorry is not threaded inside only threaded on the outside