By all means experiment on a used gun before ordering a custom gun. There are good techniques for adjusting an existing gun within some reasonable limits and reasonable costs. Read Michael Yardley's book "Gunfitting" before going further. Your Stevens 311 is a perfect, low-cost test bed. The biggest cost of a fitting is likely to be transportation to and from the fitting site. give us a clue as to your location and we may be able to recommend a local fitter.

DDA