Changing valve seals using air chuck: If I have air leaks are the heads toast?

[thecatseye66]

I'm trying to change my valve seals while the engine is out of the truck, but without taking the heads off. I haven't noticed any performance problems, just some smoke at startup and some of the valve seals were in pieces laying on the head when I took off the valve covers. My truck only has 45,000 original miles on it ('68 396).

I've searched the threads on how to do this and I'm trying to do the air chuck method with a spring compressor. I haven't taken apart any of the springs yet because even when I get each cylinder to TDC and hook up the air at 90 psi in the spark plug hole I'm getting air blowing out the exhaust port and/or sometimes the intake.

Am I doing something wrong or are my heads toast?