If your boat is a '74, has to be either a 23 or, more likely a 20. The only SeaCraft 21 was the grandfather of them all, the rare Moesly 21
http://moeslyseacraft.com/SC21Brochure.aspx built in the 60's, or the later Tracker 21 (built in the 90's?)
As a highly experienced pilot from WWII, Moesly knew how adversely weight affected performance, so he designed the 20's sort of like an airplane. He came up with a light but VERY stiff structure when the hull/stringers, inner liner and deck cap are all bonded together. He then raced his designs in the very rough offshore powerboat races in the 60's to find out what worked and what didn't. Did you remove the top cap or cut out the entire inner liner? If you remove the cap and inner liner, yes the individual pieces are fairly light, but that doesn't mean the final assembly isn't very stout.
If you want a very detailed description of the hull construction, click on Classic SeaCraft Home at the bottom of the page and go look at the Boating Magazine article on the '69 Seafari in the Literature section. If you find it hard to read, PM me with an e-mail address and I can send you a good .pdf file of the article. It contains sketches showing the layup schedule and thicknesses for the hull and stringers. The hull bottom is naturally very stiff at each vertical step, but notice how the 4 stringers, which are installed about half way thru the layup, attach to the hull in the center of each panel, locally thickening the hull in what would otherwise be be the softest spot. As a retired aerospace engineer, I would say it's a structurally brilliant distribution of material! In 1974, Potter changed Moesly's original 4 stringer design to 2 wide box stringers, which I have to believe is a cheaper but less uniform and therefore a less optimum distribution of material. However there is no history of any hull problems on any of the 20' models that I know of. Some of the later Potter 23 and 25 models have had some rare quality control issues, and workmanship on the later Tracker models doesn't seem to be up to the Moesly/early Potter standards. Don't know if it's in the Literature section, but I have a factory brochure from about '78 comparing the SeaCraft laminate schedules to Bertram, Formula, Pursuit, etc.; the SeaCrafts typically had the same or more layers of the comparable size Bertram, which were noted for being "overbuilt"!
I became a believer in SeaCraft's hull integrity after my first return trip across the gulf stream from the Bahamas in the late 70's. Winds varied from NE to N to NE, producing steep breaking seas of up to 6-8'. I quickly learned why the old salts always said "If the wind is out of any northern quadrant, stay the hell out of the gulf stream!!" Took us 4 hours to run the 60 nm from West End to Palm Beach (Lake Worth) Inlet. Our "flotila" included a Moesly 21, my 20' Seafari, and a Wellcraft V-20. The 21 had zero problems and had to slow down so the 20's could stay up with him. When we got to the ramp, my friends V-20 had virtually disintegrated . . . the seats had come apart, there was a big crack in the cabin bulkhead, his twin saddle tanks had busted loose at the welds, and when he got it out on the trailer, there was a 4' crack in the bottom of the hull!
On my Seafari, a few of the screws in the cabin bulkhead were loose, but that was it! I was totally beat, but the boat held up just fine!
As for the Aquasport, another friend of mine told me that after making a rough crossing to West End in his Moesly 21, he saw an Aquasport being hoisted out of the water with water pouring out of numerous cracks in the hull! It didn't sink because of all the foam, but it did not appear to be a very robust hull! He said there are some good glass repair guys in West End with a lot of first hand experience of which boats had problems crossing the stream. SeaCraft hulls were highly regarded by those folks! The Aquasport? - not so much. Denny