[A. Most 375's will shoot 38-55s as well. The 38-55 is slightly longer but I have heard they work in Win 94's...True?]
My early .375 BB94 could/would - but Winchester specifically warned against it, and later changed the chamber(s).
[B. ON a top eject, can you mount a reciever peep sight?]
Of course. D/T will only be needed if a TE isn't already done by the factory - but BB94's are. However, the BB94 doesn't use the samo Win 94 peep, due to the rear receiver bulge, and needs a peep specific to the gun (available from Williams as the FP-94-375, IIRC).
[C. What is the longest reasonable hunting shot one should take?]
That's an open question, and can be very argumentative, depending upon one's abilities, rifle, and ethics.
Personally, I don't take any game shots beyond 250-300 yards, but YMMV.
[Will full power loads, 200 yards would not be out of the question or am I wrong?]
IMHO, a game shot @ 200yds with a peep-sighted .375 BB94 is right at the very limits of both the cartridge's and the rifle's abilities.
FWIW - I would make the trade, since I happen to like leverguns much better than Post-74 Browning bolt rifles.
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