Those HP numbers sound about right for that era. My T120 flattrack engine was set up for 1/2 mile, running JOMO 15B cams, Chevy valves in shortened Sportster valve guides (larger brass seats), 13:1 pistons, GP carbs, 26" exhaust, and a short 4th gear. Just for comparison, that was high 13s at 5000 foot altitude on a warm day. That was identical to my built CB450 on the drag strip, which was the same bike that ran a best unstreamlined pass of 118.733 at Bonneville 1969 (but only had 116.654 average). With the fairing, best mile was 124.65, but not until I was coached by the veterans to be a lot more aggressive off the line!
Here we are 40 years later and these bikes are still about the same speeds, EXCEPT with AirTechs much better streamlining giving much more speed and being able to pull 10,000 rpm with a super short stroke. My current bike is difficult to launch because there is no way to run a flywheel with the chain drive conversion; which is a deja vu flashback to that CB450 (which couldnt run a flywheel, either!) and was very peaky. With a flywheel, that CB450 only ran 107 unstreamlined at Bonneville, in 2007, but I was off on jetting also.
You are correct; the M-PG record is 123.9, but I'm guessing Roosevelt was riding the 8-valve Weslake-Triumph. The current MPS-PG-650 record is about 133 1/2, and that bike had the AirTech Charlies Toy fairing on a nearly stock CX500 chassis with stock shaft drive and rear gear ratio. Streamlining is a lot of fun, and cheaper than horsepower! About the video...not my posting on utube. The bike only gets past 160 at the end of the 3rd mile. It has never run better than 159 mile, with either engine.
Sorry for the rambling post; trying to connect the dots between the years, and see what we've learned.
Hope this is useful to your planning and building, which is some of the most fun.
Regards, JimL