The Black Widow speaker from the factory would have been 4 ohms.
Your best bet for that amp is going to be the 1501-4 SB. This means model 1501 4 ohm chrome dust cap short basket. These are the most commonly found.
Hopefully not to introduce confusion, the very earliest Vegas amp used a ribbed cone, chrome dust cap, short basket.
These are getting hard to find in good shape.
Not to be confused with the later 1502 with the paper dust cap.
These speakers were made for just a few years, while the 1501 was issued longer, that's why I say they will likely be easier to find.
You can search the forum for some examples of these.
Some shots of the short basket ribs and magnet iterations coming shortly: 1501-4 SB shown below.
There were several variations of BW magnets on these speakers over the years but that's less important.
I can post some pictures of the chronology of those if you like.