Glenn:

Choice is much better in .366" diameter, and a .400/.360 will be more representative of original Fraser. I've never seen one in .400/.350 (not to say there weren't any mind you) but I've seen a lot in .400/.360. There's a Fraser double .400/.360 on Cabelas site now. I've also seen Fraser's in .375 2 1/2" NE. The .350s and .360s are a better choice though, as brass is easier.

The difference between the .400/.350 and the various .400/.360s, both physically and ballistically, is invisible, excepting the thick rim of the Westley Richards .400/.360. They all use the same case (you can use .400/.350 brass if .400/.360 isn't handy) with very slightly different loadings, with very minor differences in bullet diameter and weight.

.400/.350 2.75": 43 Cordite, 310 grain .358" bullet = 2000 fps/28" barrel

.400/.360 2.75" Purdey: 40 Cordite, 300 grain .367" bullet = 1950 fps/28" barrel

.400/.360 2.75" Fraser: 40 Cordite, 289 grain .360" Oblique Ratchet bullet = ?fps (but Fraser's always seem to be proved for 40 Cordite, 300 grain)

.400/.360 2.75" Evans: 40 Cordite, 300 grain .360" bullet = 1950 fps/28" barrel

.400/.360 2.75" WR: 41 Cordite, 314 grain .360" bullet = 1900 fps/28" barrel

The .350s and the .400/.360 Purdey used a max rim thickness of .050". (I always use shortened 9.3X74R, which has a max rim of .055", and have never had a problem). The Evans and Fraser .400/.360s seem to have used a max rim of .045". The .400/.360 Westley Richards used a max rim of .065". All cases are otherwise identical.

If you're looking for something representative of original Fraser with .400/.350 ballistics, I'd pick .400/.360 Purdey. There's a plethora of .366" bullets available - better selection than .358". RWS brass lasts forever - I've never had to discard one due to fatigue in 17 years of shooting mine. If you just want to have it, properly headstamped brass of excellent quality is available from Horneber. All of the above are far more effective on game that paper ballistics suggest.