Not sure if this will help you.
I made a tool that goes over the rocker shaft to do this on a small block. It works very good and easy. We had a little over 600 pound spring in there. If I needed more leverage I just use a longer tube or pipe. I made a short shaft to bolt down and the compressor slid over the shaft. Moved it from cylinder to cylinder.
Found that if the piston was on TDC that the air pressure would push the piston down, no problem, just keep the air in there until done. How much spring pressure do you have?
Hope this helps.