Here's a picture of a similar tool if that would help.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=1822672538

I think samstagsales.com sells a generic tool like this $70? you would have to rig up some type of bar across the length of the engine block to place the tool's hook under. Looks like you could fabricate a tool also maybe use an appropriately sized cheap socket and grind away most of it leaving only a portion neded to bear upon the spring. I anticipate some 603 headwork and I plan on fabricating this tool the hardest part would seem to be the bearing surface that contacts the spring it will need to be strong because much of the supporting area needs to be removed to allow access to the top of the valve stem. I'm considering use a screw and beam arrangement bolted to the head rather than a hand lever also, it might be easier to just screw the tool down on the spring let it be held in place and remove the clips than trying to remove the clips while at the same time exerting pressure with a hand lever style tool. Just a couple thoughts along this line. I'll probably scare up a used camshaft timing chain sprocket to attach to the #1 cam tower as a retainer also, should be pretty simple, the sprocket mounted onto a shaft and both mounted on a tensioning base mountable to the cam tower or even the head itself attacthed to the valve cover bolt threads from in front a simple device could maintain the needed tension.