I recently looked an early 80's Capri 25 on a trailer. The weight of the boat and the trailer combined were well within the GVWR for my 2009 Silverado Gas V8. The fin keel has the boat sitting pretty high on the trailer so it was equipped with a VERY long tongue extension, but the PO had also added a balanced lift point to the keel bolts for crane launching.
Considering the height of the boat on the trailer, its overall length including mast, I'd say a 25 foot would be as much as I'd be comfortable towing with a gas V8 truck, anything more and I'd want a 2500 Diesel. The PO had added a third axle to the trailer, I think it was overkill but he liked the way it towed.
I've already picked out my next truck, I want the new Dodge Ram 1500 that has the Eco-diesel, I ill buy it in the 'work truck' trim because I refuse to pay $50K for a truck that has a bunch of fu-fu chrome trim and other usless crap. The eco-diesel has impressive towing capability, about 9.5K pounds and gets impressive fuel mileage, far better than the 18 to 19 highway I'm getting from my V8 Silverado.