In my conversations with Blue Water Yachts and other keel restorers, there are variations in keel design and composition between production years of my V21 and other models. Yours might have been a cast iron one. Mine, which I finally was able to remove, has a long 4" by 5/8" bar running the length of the keel at sort of an angle with filler material fore and aft to make it 10 3/4" wide (up to where it flares out for the lead end) then the bottom end of the bar is embedded in a cast lead bulb. (It looks like steel was positioned with some plates extending outward and the lead cast around it to hold it all together.)
Is your total weight in the 400 lbs range to offer enough ballist and counter-weight?
I am not sure you need a zinc on the keel. If sealed completely in glass, it should not be able to conduct current. The pivot bolt in the hull is isolated from other metals, so it doesn't seem to me that would be ansource of corrosion. If the old keel rusted away and delaminated like mine was doing, I'd guess it was because the glass was damaged, perhaps around the pivot bolt or bracket for the winch cable. Others may know better.
If you were to attach a sacraficial zinc, it would have to be on the outside of the glass, seems to me!
Post pictures of your constructed keel steel core?
Here is what mine looks like stripped of all the loose filler and glass:
https://goo.gl/photos/ch2Fpo8snLzL7oZ37
Here is a close up of the lead bulb still mostly encased in polyester and glass.
https://goo.gl/photos/2WxtUbvsYwvjYzdj7
The brass ferrule for the pivot bolt was epoxied into a larger hole.
https://goo.gl/photos/gN8dbW5Hdhkk2VXZA
Good luck with your keel!