Did I say 250 HP? That must've been around Unger's campfire when the Chivas was making the rounds!
The only credible dyno tests I've seen were published in the May 1955 "Hot Rod" magazine and reprinted in the March 1983 "12 Port News". The tests were run on the McGurk dyno and started with a bone-stock 261. From a base of 114 BHP at 3500 RPM and 201 lb/ft of torque at 2000 RPM, they went through a series of modifications and ended up with 197 BHP at 4500 RPM and 268 lb/ft at 2000 RPM. This final reading was obtained with 287 cu. in displacement (from a .125 overbore), slipper skirt pistons, Corvette dual exhaust, dual point distributor, a seriously reworked head & valvetrain, triple Stromberg carbs, and a real snorty "super 3/4" grind McGurk solid lifter cam. It was described as being "on the ragged edge of streetability".
Obviously it's hard to critique something published 50 years ago but I'd say the numbers are pretty close. 235/261 hop-up parts being about the same level of technology, I wouldn't expect much more these days. Unless you've a got a big budget for one of the new Wayne CNC alloy 12 port heads, that is.
