In my Haynes manual it says the SAE reading should be 118hp at 5800 and DIN 100hp at 5500. What's the difference between these two readings and how do they compare with whatever method we use today for measuring output?

In a nutshell, SAE is for the bare motor without any parasitic loads, while DIN is for a motor driving a fan, alternator, waterpump, and whatever else is hooked to it. The actual units of HP are the same -- DIN is just the more real-life measurement.

Of course you were measuring HP at the driven wheels, which is always much lower because there are friction and loading and inertial losses in the driveline. You wheel HP and torque numbers are excellent.

Yeah, a couple of my runs start at a little over 2k, but by the time the engine get's up to full power it's right where my max trq. ratings on the sheet are.

Think about this... you're measuring how the engine accelerates. DynoJets can't measure steady rpms, only how fast rpm increases against a load. The chart shows the result. If you'd started from higher rpm, the peak values would still be the same (or maybe lower, as the carbs would have less time to respond before hitting the peak torque and the mixture might be further off).