The chamber length is in the barrel. Don't know why you would want to rifle the barrel - probably more $$ than the gun is worth, and then it is limited in use. Don't know why you would want to cut the barrel down unless you are attempting to make a home defensive weapon out of it - and there are a lot better options than that.
The barrel you see on Gunbroker may be a rechambered (reamed out) barrel for an earlier gun (the "mark I" or Mark II 1400) or it could be a barrel for the later Model 1400, but I don't think the barrel interchanges with the earlier 1400s ('Mk I' or Mk II).
You don't say if your gun is one of the Mk I or Mk II guns or not. If a 1400 Mk II - it only holds three rounds, one chambered, two in the magazine. There is no plug and the magazine tube cannot be expanded. The Mark II Model 1400 replaced the 1400 model (called 'Mk I' but it never had that roll mark) in 1968 and was discontinued in 1973 - so your gun is somewhere between '68 and '73 if this is a "mark II" 1400. There were 'model 1400s' made again in 1989 - but not the same as these early "mark IIs".
The earlier 1400s are good guns - the only real weak spot is the plastic follower - very easy to break. Excellent bird hunting gun, probably not something that would stand thousand of rounds on a clays field.
