A good one to use is the Browning BSS. They are quite strong although there is no top fastener; and also fairly common, and realativly inexpensive. However, by the time you are finished with a reasonably good job of conversion, you will have spent almost as much as you would on a decent British BPE rifle. Your conversion would have little resale value unless very well done, whereas an original will always have value to someone. Most conversions I have seen are very heavy relative to caliber; that is unless you are building a .577 or .500 nitro.

All the best, Mal