Valve Spring Removal with Heads in Place?

[Frizzle Fry]

I picked up a few parts to do a little "bolt-on" improvement to my 396. I got a cam, intake, carb, springs, lifters, pushrods, timing set, and water pump.

The problem I'm running into is changing the springs with the heads in place. I planned to use the air-compressor connected to the sparkplug port to hold the valves up, and a lever-bar from the rocker studs to "pop" the retainers/inserts loose. Well, the valves just open when I apply pressure with the bar. I think I'm running about 90 psi on the compressor, so I'll try more pressure, but if that doesn't work, I think I'm stuck.

I did'nt want to remove the heads, but I suppose I could. The question is; are there any other ways to change the springs with the heads in place? And/or should I just remove the heads to do it or simply use the old springs?