"That was real horse power back then:" Was it really?? Read on and view the sources..
In the 60's up until 1970 (ish) the US auto manufacturers used "Gross" hp ratings. Which meant they took an engine, put it on a stand without any compressors, air-conditioning, anti-smog devices and exhaust system. They measured at the fly wheel, (HP = 746 KW) p/s.
In the 60's as the engine displacements got bigger and the horsepower wars started, it wasn't uncommon for each manufacturer to do their own testing and release their own numbers for marketing. As the 60's progressed and HP ratings started increasing, pressure from safety advocates and insurance companies put pressure on the companies to lower the HP. It wasn't uncommon for the manufacturers to not make one change to the engine, rather release lower HP numbers. This was called "deliberate under-rating". GM was the biggest violator of this, and actually put a hp per pound ratio limitation on their designers.
(2) In 1971 California enforced all the emission standards and reporting of Net HP, SAE standard J1349 testing protocols for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold. Thus, vast decreases in reported horsepower.
(2) In 2005, SAE Standard J2723 was enforced which meant that the testing standards and the testing itself was done by the manufacturer with an independent observer and certified at an ISO9000 testing facility. While it's true the test is done at the fly-wheel, vs. crank, the numbers from 2005 on are very accurate and must reflect all variables such as how much oil was in the crankcase, engine control system calibration, and whether an engine was tested with premium fuel.
So, while you state "that was real horsepower back then" - in actuality was it really?
Secondly, "a 400+ hp truck today won't do much more than a 200ish hp truck 20 years ago" is highly an unlikely statement. Gross HP was rated with no emissions, but the equation of 746kw per second is pretty much a standard, however, I'd much rather take a new 400HP truck, given direct inject, ISO9000 testing along knowing that all tests were performed post emissions. You'll probably find that the 200 hp truck (20 years ago) was probably lower than 200 hp by modern SAE J2723 standards.
"The HP may be there on paper, but in the real world, where it matters, it's a different story" - The only thing I can think you mean here is that HP on paper is not what is at the wheels. Which is a true statement, all testing is done at the fly-wheel. Which is why a Tesla making 700 hp, is REAL hp. The electric motors are at the wheels, "instant torque". Which is one of 'many reasons', why super-car makers like the new NSX (and I think the new Audi R8's) are doing torque vectoring with electric motors at the wheels.
(4) If independent testing with industry standards rate a modern day engine, at the fly wheel, say an EB 310 hp at "4500" RPM, it is a true test and real numbers. However, what is "real world"? More likely, it's 272hp at the wheels on a dyno (3:12 minute mark)
https://www.youtube.com/watch?v=vrqbaORCv2Q
(3) So while there is a discrepancy of 30 hp from the SAE J2723 vs engine testing to wheels, if you really need to eek out that 30 HP, it can quickly be done on an EB Mustang with a tune, new down-pipe and a few other mods... A fully loaded vehicle, all the trimmings (cooled / heated seats, remote start, sync-3, track apps, track and sport+ modes) in a great looking vehicle pulling down 300 hp, tune-able to who knows what, all under $30k and gets 30 mpg's (actual mpg's I got from Phoenix to Flagstaff). What isn't to love?
"Not sure what changed" - a lot! Computer designed engines for maximum power and fuel efficiency, direct injection, software driven variables, sensors, switching sensors for fuel, oxygen, etc.., ports for computers software variable tuning, also giving more variables to the drivers.
http://autoweek.com/article/technology/what-direct-injection-autoweek-explains
View source #3 - the proof is in the pudding!
Sources:
1.)
https://ateupwithmotor.com/terms-technology-definitions/gross-versus-net-horsepower/
2.)
https://en.wikipedia.org/####/Horsepower
3.) American Muscle EB Mustang & BAMA Tune Dyno testing and track times:
https://www.youtube.com/watch?v=l0iR59nRue0
4.)
https://www.youtube.com/watch?v=vrqbaORCv2Q
Hope all this helps, happy motoring!