The knocking could be because the angles are wrong but it could be something else entirely. Usually bad angles on a drive shaft will cause a "shudder". The shudder is essentially caused by the u-joint oscillating and the speed of the drive shaft constantly changing slightly as it rotates. If that shudder is shuddering against some loose parts or some excessive slop in the slip yoke I could see how that could cause a knock.
Not to pick apart the shaft in the photo but I see where a new weld yoke has been welded in one end but I do not see any new balance weights. Though they could be on the back side of the shaft where I can't see. I also see a stock balance weight still on the tube. This leads me to believe that the shaft was not re-balanced after being modified which would certainly cause some of the high speed vibrations you are getting above 40mph.
I think you've probably got a multifaceted issue. A little bit of bad angle, a little bit of bad balance. You can read more about how to diagnose some of these things here
https://4xshaft.com/blogs/faq/diagnosing-drive-shaft-vibrations?_pos=1&_sid=d6ec76a03&_ss=r.
I agree with everyone who's told you that you should do an SYE and new drive shaft but in the meantime you might want to try rotating the pinion down a few degrees. 3-4 or so but it almost doesn't matter exactly how much. If bringing the pinion down helps, or rather if it affects the knocking at all by making it better or worse, you can be confident you are not barking up the wrong tree and that your issue is most likely angle related. If it doesn't have any affect on the knocking, you may want to look closely at some other peripheral things. I hope/assume you have adjustable upper control arms as I can see that the lower control arms are still stock. If not, you may be wasting a couple hundred bucks on adjustable control arms to see what happens when you bring the pinion down and would be better off forgetting about pinion adjustment and just doing the SYE and double cardan shaft.
