Aside from all it's issues, the rifle is VERY heavy. Heck no, I finally got the boys in Utah to fix me up with a 98 in a 91 stock and got a bit of refund as well. I still have the 98 and it's a great rifle. Even the .25 ACP would probably be too long for the White breechplug (andy you are familiar with the White 209 breech plugs). Take a look at one of yours and see what you think. Doc thought along those same lines but had to Doc'izeand so created the 336 ignition system, which does help immensely I'm told.
His version uses the .32 S&W Short case which is about 1/1000" shorter case length than the .25ACP but perhaps a better rim. I don't know if that plug has a concave on the powder end but you can see there isn't much room left for metal along the flame channel.