Thanks Jay ~ and others, As several people had recommended, I do already have the Nitto pry tool. The issue I was mainly concerned about with this particular uncommonly wide old 1950s GB stem and a bar with a very shallow drop and severe bends was simply not wanting to force the stem open just a wee bit "too far" - for fear it would snap at the thinnest point in the band of the clamp area.
However, urged on especially by the recommendation of List member Phil Sieg [Thanks buddy!], who had also grappled with these very same stems himself, I did indeed take a shot, and sure 'nuff the old alloy stem held just fine.
I would also like to recommend to readers another tool I have which works extremely well for this type of brute force application. Technically, it is a circlip-type retainer ring spreader. This is basically a plier-type tool which operates in the reverse; when you grip down on the handles, it forces open the jaws. This particular hand tool works in openings from around 3 mm to as much as 20 mm. The outsides of the flattened tapered jaws are knurled in a file pattern to prevent slipping.
This tool also works extremely well to open up the slot in a seat post lug cluster by simply spreading the binder bolt "ears" apart as much as needed. Very handy if someone had previously tried to use a far too small seat post and had crimped the the seat tube slot and opening down too far. Works better for me than trying to wedge a narrow screwdriver blade into the slot and just pounding away; gives more control while using far less force.
Thanks again for supporting this too timid tool wielder. I had visions of making stress fractures which would unintentionally create my own "suicide stem" - but apparently this was not at all warranted.
Cheers!
Bob Hanson, Albuquerque, New Mexico, USA