Been down this path a few times on mine and I am building another at the moment.
My method is to loosely assemble the valve rocker gear without the clam shells. The trick is to ensure the spring tangs are located in the small holes in the angle bracket whilst ensuring you get the correct rotation of tension in the spring. Its not straightforward and a number of attempts will be required. Best to support the timing side bracket in a vice for this to keep stable and have hands free.
Hard to describe but get to this stage without the angle bracket fitted but use it to locate one tang of the spring and rotate to get spring rotation and loosely assemble with one nut giving enough clearance to get over the other spindle. Then by magic use a screwdriver to push down the tang to locate the hole and try to locate.
Offer up the lower clam shell and at the same time rotate the rocker arms to imitate their final operation. If you have a helper available do it. This allows you rotate the rocker arms and put tension on the springs and allow fitting of the upper clam shell. A loose fit of the end nuts is required to get the clam shells inside the assembly. If you can't locate the tang at that stage the cutout in the upper clam shell will allow you to locate the tang (with screwdriver assistance), Do this when you have screws loosely fastened in the upper clam shell. If your OK at this stage fully tighten the screws. The angle plate can be removed of fitted with the tangs in the correct position.
I have one at first fix at the moment which will be dismantled shortly for final clean and reassembly and grease.
The collars must have generous chamfers on their mating surfaces with the spindles. I'll dig out my notes on rocker arm clearance / movement to expect from memory its around 0.125 inch average but I'll check.
54 Square Four, 1934 RH500 twin port, 1936 RH500 single port, 1945 RH500 (1936 ) twin port, 1947 VB Buttermilk.