Not to worry, they are easily fixed. A reseal job and it will be good for years. You are right, the Sheridan is a powerful (and accurate) rifle in it's own right even without a "steroid" job. Don't sell the old intermount short. I have one on my '67 Blue Streak and it works fine. I don't believe they will cause any problems as long as you don't tighten the screws to much. Just *snug* and you're good to go.
Kind of sounds like the 312 you have. Soak it down with some penetrating fluid. Tap on the wrench while you try to unscrew it. That's all I can think of.