If it were me, I would replace that swim platform with a newer, plastic/poly style. There should be models with a lower, shallower profile.
I had to raise one on a friends 22 Seafarer about 2 inch's to install Bennett tabs. Sealed the holes with Lifeseal and finished it off with Marinetex... worked out fine.
Also;
http://www.bennetttrimtabs.com/faq1.htm
Check out number 6.
Bennett does offer a shorter actuator, (2 inch's shorter). Not sure if that's what's used on the sport tabs, (M80's, M120's). I would contact Bennett and find out -
http://www.bennetttrimtabs.com/help.htm
their customer service is outstanding.
I seriousely don't think you need 18" tabs. I've had the standard 9" x 12"s on a 20', 23' and 24' (Grady's)... they worked fine. The Seafarer mentioned above was also 9" X 12". The M80's would also do fine... maybe stretch it to M120"s.