where did the 70s take the horses

Net horsepower is not at the wheels, its still measured at the flywheel it is just with accessories like alternator, mufflers, air cleaner, fan. No where near 75hp worth of drop. If it included the transmission and axle then maybe but it doesn't. It was all car makers could do to keep up with rising emission standards into the 80's and they had to make terrible choices just to get a few numbers on an exhaust sniffer. Good news is it can be fixed and there won't be much of a difference between a 68 and a 74 318, pure stock measure both at the wheels, installed in the car and the 68 will make much more power. The best stock 318 as pointed out was the 92 up, they finally got emissions figured out, came up with better exhaust manifolds and heads and put a bigger cam back in it. Drop a 92 or 3 magnum 318 in there and you will really feel a difference.

PS: one obvious way to tell NET ratings were not done at the wheels is they would need different ratings depending on manual or automatic. It was actually misreported in many magazines in the early 70s NET was measured at the wheels but they had to correct it later.