Tink,
That dovetail fitting you have on there is a definite no-no. Lash ups like that are courting disaster.
The difference in size is way too much for a gib strip. The way it is, it would be liable to come off at any sign of milling pressure.
If you want to use it on your lathe, you will have to fill part of one side of the dovetail with a bolted in and rigidly fixed filler piece to reduce the larger size down to somewhere near the smaller dovetail, then make a small gib strip to fit.
It is not the sort of job you should attempt to do without the correct experience and machinery to do it.
Sorry to be the bearer of bad news, but in this case, and as always, safety comes first. We wouldn't want to see you getting hurt.
Bogs