I am always wary of any metal ‘embedded’ in fiberglass. This is nearly always a recipe for later disaster.
The correct way (and a decent fix for you) is to drill right through the boat from outside. If you drill through the plate, without removing it, so be it. Locate the holes inside and lay up some extra ‘glass - going wider than you think you need to. I wouldn't bolt a prop strut to anything less than 1/2” of glass that extends a good 4” or more beyond the bolt holes.
Then use a strong fiberglass backing plate (see McMaster.com) like 3/8” thick, NEW bronze bolts (NOT brass - see BoltDepot.com), wide bronze fender washers and bronze locknuts (or lock washers if you can’t find them and use ‘large-pattern’ hex nuts).
And - in spite of what anyone else here says - assemble the whole thing smothered in 5200, which is the only thing the factory would/should use. The 5200 will hold better than the bolts do (it’s what holds your keel on), will not fail underwater and admit water to your newly-constructed structural sandwich, and will provide the required vibration-dampening flex without breaking its seal.
I have done this job often in production. And I know I get known as ‘Dr Doom’ on here; but it’s because my standards are noted in the industry for being consistently higher than those of people who think an amateur’s assumption of ‘good enough’ is good enough for a space capsule taking you into an alien environment that can kill you (i.e., a boat going to sea). There really are some things some boat factories get right; and one of them is a 50-plus-year track record of relying on 5200 for structural integrity.
Backing plates ‘embedded’ in fiberglass is how motorboaters mount their seats’ bases, which accounts for why (with the typical motorboaters’ fetish for silicone) most of them over 20 years old have rotten cockpit soles. I could explain why they think this is the only way (since it’s almost 100% universal); but the answer would come down to poor design, poor production techniques and a lack of caring - none of which any sailboater should embrace, tolerate or deserve.