Bidhunter50,
No, for your intended conversion, finished weight certainly doesn't need to be 13 pounds; 10.5 or so pounds is just fine. I have several fine British double rifles in .577 BPE, and they weight right at your projected weight range.

Yes, some of the Spanish 10 gauge 3.5 inch magnum boxlock sxs doubles had Greener crossbolts, but some did not.

I leaned the double shotgun to double rifle conversion process from W. Ellis Brown, in gunsmithing class at Tech College in MN, prior to his writing his book on same subject; I follow pretty much his whole process. I strongly suggest that anyone who is anticipating doing this sort of conversion work to get and use his book. Regarding proof load for cartridge you plan using, 650 grain bullet @ 1800 fps, determine what powder load you need and will use to regulate the rifle, and obtain bullets 10% heavier than that--THOSE WILL BE YOUR PROOF LOADS, not your normally used service loads. If you need specific loading data for your service loads, send me a private email, and I will look up my loading data that you may wish to use. As to breach pressures of the .577 BPE, 650 grain bullet, I don't have it at hand, but do have that data in my reference material, so if you must have it, send me a PM. I will need time to look it up. Yes, beyond doubt, for sure, use dacron fiber filler for filling excess space in cartridge cases----I have used it for over 50 years in various loads, full nitro and nitro for black, and it has always worked for me!!