This is what I came up with to remove the valve springs with the cylinder head on the crankcase.
How you keep the valves up is up to you, I am thinking the 'rope in the cylinder and the piston pushing up'.

this trick has two stock rockers with each of the rocker arm side spacers trimmed.
I set these close together as I am using it to work with beehive springs and retainers.

Photos are of a bench test, and it worked really well.

I went with a jack screw design, that allows one to compress the spring and have the screw hold the compression with out holding a lever.
The lower base supports the jack screw. Used two 7/16 bolts - nuts. Filed the 7/16 nuts to make them a snap fit into the cylinder head bolt holes.
Used a carriage bolt as the pivot point as the angle of the rocker arm changes as the spring is compressed. The radiused head of the carriage bolt
sets against the rocker rail and is fully supported.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

with ratchet

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

How about piston at TDC and air pressure?

My compression tester has a quick disconnect fitting on it...............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I've done it that way. You need a very secure way to hold the crankshaft from rotating - which it will try to do with a LOT of force.
If the push rods are removed it doesn't have to be at TDC.
Rope is safer. Also if something goes wrong the valves can't drop into the cylinder.

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The idea of TDC is so if the valve drops, it doesn't drop all the way thru.............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes