Your overthinking the pinion and u-joint angle theories as relating to wheel-hop; these devises and their relative positioning have more to do with their mechanical survivability, not at leveraging the rear differential in an attempt in damping unappreciated movement. 
As stated by others, the u-joint manufactures want some angle deviation in order to create rotation of the rollers, this so as to induce movement of the lubricant and to resist the tendency for the rollers to state the cross-trunnions. Generally this being something of the range of 1° - 3°; greater value sums in the 3° - 5° are still generally considered acceptable with as expected lifespans of in-service usage; but after something approaching 7°, or more, anticipate a reduction in service life.
Applied torque loads are better handled with lesser angles, so in high load applications the straighter the drive-line is to the out-put shaft and pinion gear stem the lesser are the forces on the u-joint and lesser are the drive-line parasitic losses in performance delivery. Therefore, generally the intention is to have the drive-line in the straightest alignment possible at the highest torque loading instance, this being of when the vehicle has loaded the rear axle "on-the-leave", as this is the most abusive period for these parts.
Now, there are considerations for pinion-angle relating to aiding in loading the rear suspension and increasing the "bite" (tires), but this mostly is of greater concern, and reeling results in lighter vehicles, particularly suspensioned Altered and Dragster cars.
"Shocks" would probably be better understood if we in America would utilize the proper vocabulary in their identification, this being "Damper". This as they are intended to "dampen' motion; they are not intended to "arrest" motion completely. the later is better made the responsibility of that which is intended to locate the axles' position, and let the shocks do that for which they were intended as a suspension motion control devise. 
And "some" weight in the car, shouldn't change the pinion angle in its' relationship to the mainline of the power-plant & transmission dramatically, as this is generally a consideration by the O.E.M., but it will change the driveshaft and u-joint angles.
Scott.
Scott, I disagree with angles of 5 degrees being acceptable. Build it if you want, and some low speed, short wheelbase crawlers may be forced to live with it, but not for a 50+ inch shaft. I have not seen Spicer or anyone else recommend anything like that. I do agree the primary purpose for the degree of angle is durability and proper movement of the rollers, with parallel operating planes planes under load for efficiency and smoothness, but it is not the only factor. However, I do agree and thought that me mentioning that 3.5 had no bearing on wheel hop made that clear
Second, weight in the back
shouldn't change things at the pinion, I agree, but often it does. Very hard to believe you think that Ford got this right for a 500+ HP engine in a likely lowered car, as no OEM I know has gotten it right in that era, coil or leaf spring. Regardless, my recommendation was looking at all angles with the car loaded, not just pinion angle instead as a response to the 3.5 degree angle he reported at crank centerline. You stated as do I, it will change and HAS to if the rear axle rises. Will the pinion angle change? maybe, but it definitely twists under load.
Additionally, that angle can matter, as "how hard it hits" can contribute to "how hard it loads and unloads"
Finally, of course the primary force causing wheel hop is torque wrap up and unload, however, there are forces call secondary, tertiary, or even better just call them contributory. There are other forces at play, one of which is gyroscopic. Nowhere did I say it is primary, nor leverage, but it does exist
Now, after a little wasted time bantering. One thing I would look at are the spring perches, especially if this is new behavior. Aftermarket housings are far less susceptible, but stock perches are not too great, and more often than not, broken when you get any real power. It would not be unreasonable to believe one or both could be broken, but held together with the U-bolts. I can't be sure it would hop, but it sure wouldn't launch right, and worth checking for no other reason that the hop could have hurt them
In the end, without seeing what is happening, my assumption is the main leaf is the culprit, but not only the front, I have seen clamped front springs wrap in the rear as well and open between leaves to look like a bird spreading it's feathers. If the perches are good, potentially verifying all spring clamps are good and in the right position to mitigate some of the movement as well.
I am still not convinced that your angles are right under load, but given this response I have to be clear that I do not think that is the
cause of hop
