Welcome to the site Bruza. I called around looking for that trim piece/retainer, came up too late. Friend had a '58 Pontiac Chieftain parts car but already took what he needed and sold it. I can't find a pic of that retainer anywhere.
I've installed at least 10 windshields in 58 chevy/pontiacs, and for the life of me...I can not remember how they were. All those old days are blending together on me. I do recall one had a thru-bolt retainer into the A pillar, but not which model.
Don't over look an Impala retainer, although the upper and side windshield mouldings are of different size...Gm usually used the same lower moldings. The corner maybe slightly sharper due to the shorter windshield, but the retainer is probably the same. Pontiac counter models (Chieftain, etc) will be same as chevy.
The tri-5 Chevy lower retainer maybe an option if you had one to see if it would fit into your trim (or maybe modify too). But tri-5 had a dog ear on the lower corner of the trim to screw to A-pillar door jam.
If 58 is a thru bolt A-pillar retainer, you may be able to use a large body/rocker trim spring clip. Gm, Ford and Mopar used these during late 50s-60s.
Wish I could be of more help.
Your trim piece pic for those wondering
Tri-5 retainer
Trim spring clip (thru-bolt)