Tappet block question

I'm familiar with some of the differences 1948-1980?:
iron vs. aluminum
tapered head 1/4-24 screw vs flat head 1/4-20 screw
Palmer suggests pan and shovel blocks are not interchangeable.
Unless he's referring to the pushrod tube seal area, why not?

Panic, as I recall, that is where the difference is. Used to be a problem at custom shops where they did chrome exchange, you'd get shovel blocks for your pan blocks and the tubes wouldn't seal right. However, it didn't happen to me, just one of those story's I heard passed around.
DD

Isn't there a difference between the '48 to '52 with the hydraulic push rods and the '53 to '83 with the hydraulic lifters ?
Something to do with the oiling system passages' to the lifters ?

Thanks.
What I'm trying to pin down is how many different angles were used in the entire big twin (except VL, UL) history.
These I know are all different:
Knucklehead
Panhead, shovelhead
Evo
TC
But... people have run Evo tappet blocks in shovel motors (although the word is slowly getting around that you need the Evo cam as well), Evo rockers in shovelheads, and TC rockers in Evos so evidently most street engines are very tolerant of weird cam timing.

H-D made all 4 knucklehead rockers different (partially) because the distance from the cam to each valve is different between front and rear.
H-D used only 2 panhead & shovelhead rocker shapes.
The question: did they also use different angles inside the blocks, and (if yes) is this different between pan and shovel?

1.43 to 1,is the ratio for Shovel Head rocker arms.
The ratio of 1.5 to 1 for Pan Head rocker arms.
I have been told,that there is another ratio for Pans.
Maybe the same as the Shovel, this was used on the '65 Eltra Glide. That would be the 1.43 to 1.
If any one else has rocker ratio info please post it.
I have tried to find any differance in the angle in the block bore, and I can't see any.
Have checked early Pan(aluminum)blocks against later steel Pan, and Shovel blocks.
The only Evo blocks I have are in use.