P.S. Norm, most people would go for a guide that is a nice, firmish, sliding fit to reduce twang. Not so tight that you have to hammer it in, though. The diameter you mention may be a little loose? But, if you don't mind going "old school", moly grease will help dampen the twang. Plus that extra length will help.

I'm guessing you may have to go for the original type of guide fitting, ie into the end block, as a guide with a flange at the rear may not allow the piston to come backwards by the required mount to allow cocking? You'll need to have a little play to see if this may be the case. If a design with a flange would work, you'd need a wider diameter delrin bar to start with, larger diameter to suit the cylinder ID and the smaller diameter to suit the spring ID. Does the piston have a latch rod or does the trigger sear engage with a cut-out in the piston body?