I think you do have cause for concern with a chainplate such as that.
With the uppers and lowers on that same deckplatetang,set at athwartships , it can and will see-saw on the fore and aft chainplate welded below it. The 'captive' deck plate helps resist fatigue from working, but the welded design is to be avoided when possible. This chainplate arrangement is done on many production boats,with either a rod or chainplate or cable, in answer to the same interior decorator needs as your 303, and it does fail more easily than a straight piece of flat bar would.
Maybe you could carry that fore and aft flatbar chainplate up through the deck and weld a deck plate around it. Perhaps just adding some gussets to the existing set-up would be enough to increase resistance to working and add more weld area, for strength
"Nothin' too strong ever broke." -Maine proverb (thanks Brion)
Have fun with your boat