1956 265", 240 horsepower:
9.25:1 compression, 2 WCFB 4 barrels carbs on low-rise aluminum manifold, full split dual exhaust with the first Rams Horn style manifolds, small solid lifter flat tappet cam. Heads were the same Power Pack versions used on the 265"/225 hp.
The 240 hp engine was basically the 225 hp engine with a cam change is all.
Cam used was the first "Duntov" cam, also commonly known as the "097" cam by the last 3 digits of the GM part number.
Solid flat tappet - 287/287° duration, 228/230° duration @.050" lifter rise, .394/.400" Valve lift, 110° Lobe Separation, and .012"/.018" valve lash...... at least this is what modern catalogs show. I've also seen .398" lift listed along with .404" lift.
My old man had the 097 cam in a 327 back when I was a kid. It was fairly gnarly for a family-driven streeter. Hell my Mom drove me to school with that car. Definitely preferred 3.55s or 3.73s with an M20. If I was doing it with a 265? I'd use a close ratio with a 4.11/4.30 or more
It’s easier to get power into the range of a horse a cubic inch with a small engine than a large one without resorting to some means of boost. The big reason for this is as engine displacement grows it is difficult to grow intake system flows as quickly. Other dimensional things like the choice of valve angle to bore center in this case 23 degrees are innocuous at 265 inches but become an impediment at 350 to 400 inches requiring a lot more work in the design effort to get improved performance. You will note that the Gen III engine started at 15 degrees, something thought of as racer extreme head’s for the GEN I and II engines. Ford ran 9 degrees on the early cam in block Indy engines in the very early 1960’s. So while this angles effect on power was well known, it was a long time coming on production wedge engines. However don’t confuse this valve in chamber angle with chamber-less head’s as having no angle between the the valve stem axis and that of the bore such as the 348, 409, 427. There the block is cast to provide this angle.
Be careful also with the "240 HP from 265 ci" thing. Engine power is rated way differently between the fifties and nowadays, and a '56 265 is certainly not 240 producing modern days HP!
In the fifties, engine HP was rated gross, measured at the flywheel without accessories; today, engine HP is rated with all accessories, at the wheels. Although all this varies between manufacturers...
The difference between gross and net is often accepted to be 1/3 to 1/4 smaller number for the net, but there is no exact rule. This means that your 265 may be around 160 to 180 "modern" HP, not so exciting, isn't it?
Also, for many years, the factory power numbers were just fake, decided for marketing reasons, for instance (in any given years, not just in t956) the engine in the Corvette had to be advertised as more powerful than the similar engine in the sedan...). So, all we can say for sure about your 240 HP engine is that it is probably more powerful than the 225 HP engine, and cost a few dollars more... After that...