Oil drain bolt leak

I have a 78 Ski Nautique with the 350, I have been dealing with oil leaks for a while, first was noticeably the valve covers so I have new ones to install. Upon further investigation I found the drain hose kit for the oil is leaking. Tried a few things and still leaks. I prefer to pump the oil out anyways and wanted to replace the drain kit with a drain plug. This has been very hard to find, I can't seem to get any parts store to match it. Anyone know what size the drain bolt would be? I can order a new drain hose kit but I don't use it anyways so trying to clean things up.

KENO wrote: Since GM hadn't really gone with metric yet in 78, you should have a 1/2-20 drain plug bolt. If the threads in the pan are messed up causing the leak, you can get slightly oversized self tapping plugs at most any car parts outfit. Dorman is one company that comes to mind that sells them. If it's been replaced and it's a later pan GM used a M12X1.75 drain plug bolt. I'd look as close as you can down there in the bilge to see if there's a crack in the pan in the vicinity of the threads too

I pulled out my micrometer, and the banjo bolt that the drain hose connected to is a 1/2" - 20, but there is a reducing bushing that this was threaded into in the pan. I removed that and it is a larger. Is that made to stay in the pan and thread in a new 1/2" - 20 drain plug?

The oil leak was noticeably from the hose fitting, I would like to eliminate that before worrying myself with a pan leak!

I love these I have them on several vehicles and had them on all my boats.
Fumot drain valve