68 Barracuda Alternator upgrade.

I did buy a volt meter, will try.
OK, if you have Sunpro or similar volt meter in a can, you'll find that screen weighs next to nothing and is captured in the can. So the 2 tiny bits of plastic that the screen attach screws go into are sufficient. Your triangular OEM screen is larger/heavier and more or less floating in this housing, nothing to prevent sagging and/or eventual breaking plastic.
When I would do this conversion as a service for others, I added support gussets to the sunpro gauge frame. First to satisfy myself, repeats to ensure happy customers.