Going through Fistiana. Here's what I can find "Gilbert.-bn. by Harris, Jack" There doesn't seem to be a date given. "Harris, Spaniard. -b. Cox, 40gs. 55m. Lansdown Fair. September 15, 1792 -fought with Jones, Tom, for a purse, and bolted with it, 20m. Landown, Aug 12, 1795"
That Jack Harris is not listed in Fistiana. The lives and battles of the champions of England - Ed James, 1879, page 8 HARRY SELLERS. After Sellers' defeat of Corcoran in 38 rounds, by which means he became champion in 1776, he defeated Joe Hood for J50, at Ascot Races, June 4, 1777. In turn he was beaten by Duggan Ferns, Jack Harris, and Harvey (an Irishman). Famous Fights Past & Present (Police Budget Edition), No. 54, page 31 Sellers, Harry, b. Corcoran, 38r., 100gs., Crown Inn, Stains, October 16, 1776; b. Hood, £50, Ascot Races, June 4, 1777; b. Hood, near Ipswich, July 2, 1777; bn. by Duggan Fearns, bn. by Harvey. Beaten by Jack Harris, but date not known. Harris, Jack, b. Harry Sellers. No further record known.
So it seems the newspaper writers looking for something on Jack Harris, found stuff on the Spaniard, got confused, and went with that. I'll edit the OP, then. For future reference the OP originally read I've changed the title two, which originally read "English Champions: Jack Harris "The Spaniard" (doubtful)"
Yes, Rob Roy of Boston Post, obviously confused Jack Harris with Spaniard Harris, the latter being the closest Harris in time to 1780. I have no idea what Spaniard Harris-Jack James bout he is talking about, though. Like I said above, Jack James fought about half a century earlier.
Any idea who this guy is? Oxford Journal - Saturday 29 December 1781 A Bath Correspondent says, that Ring, the famous Bruiser of Bath, who has made two of three successful Journies to London, to justify his Claim to the Title of first Man on the Boxing List, is matched with one Weaver, a shoemaker, at Tivertoe, to fight for a considerable Sum in the Tennis Court of Warminster, in the Course of this Week- The Odda at present are in Favour of Ring; but Weaver being a determined Fellow, and Superior in point of Weight, the Issue becomes doubtful.
Fistiana, 1868 edition 1719.--Figg. 1730.--Pipes and Greeting. 1734.--George Taylor. 1740.--Jack Broughton. 1750.--Jack Slack. 1760.--Bill Stephens. 1761.--George Meggs. 1764.--Bill Darts. 1769.--Lyons. 1777.--Harry Sellers. 1780.--Harris. 1785.--Johnson. 1790.--Ryan (Big Ben). 1792.--Mendoza 1795.--Jackson (retired). 1803.--Jem Belcher. etc.