You can change valve springs while the heads are still on the car. Mancini racing, and others, sell a tool that bolts onto the rockershaft pedestal. You could probably rent or borrow one too.

The trouble is when adding more lift the "valve retainer-to-guide clearance" MUST be checked. Those cams, anything above .440 lift as far as I'm concerned, are in the range of such a said crash. I just tried that on a set oof 318 heads, the retainers crash at .465, the cam has .477/.507 lift, .050-.100 clearance is recommended. It's either take the heads to a machinist for a "little off the top" or buy a self-centering tool and do it myself.

The factory valvesprings are VERY weak and would be all done anywhere between 4500-5500 and due to destructive harmonics it breaks down from there very quickly. Which can get into a situation where the valves are still open when the pistons are at TDC (Read: another crash).

It's the old "might as well situation", so do it right and do it once. A cam, valvesprings, pc seals, and headgaskets aren't that much.