If the bulkhead in your picture is the same on your tank now, take it with you to grainger & match up the OD (outside diameter) of the make (external) threads. If you are still unsure weather it is 1" or 3/4" after that due to them possibly having the same OD (which I doubt they are) get the 1" and a couple 1" x 3/4" bushings just in case, then you are coveted either way. Also, you can adapt to the bulkheads you already have but it would most likely be easier to put in new ones. There are fittings however that would allow you to go from hose barb (as pictured as the bulkhead you have now) to either MIP or FIP ( male iron pipe / female iron pipe). You then would add a MIP or FIP x slip adapter. I strongly suggest you go the new bulkhead route tho. Also note that just because I used iron pipe in the mip/FIP description that only refers to style and not material, i.e. PVC
not steel for this application.
I meant male threads not make threads in the beginning, dang auto correct!