Before we predict how Marvin Hart would do against A, B, or C, we need a clear idea what Marvin Hart was!
Sparked out in less than a round by Wild Bill Hanrahan...beat an aging Gus Ruhlin a few weeks after Jim Jeffords turned the trick...awarded a controversial ref decision win against an up-and-coming Jack Johnson, despite getting "badly punished" and having his face "beaten to a pulp," in front of a white pro-Hart crowd...Lost to 168-lb Jack Gardner... Fought an aging Choynski to a ND that seems to have been changed to a win on boxrec...Not sure what you're looking for here. PS - Completely irrelevant but Hart possibly had the least impressive pectoral development that I've ever seen in a large professional athlete.
Hart was rough mf'er. I think he gives Leon all he can handle but likely loses... Then again, there's a lot of different Leons than might show up and Hart could certainly beat many of these.
I'd like to see some footage of Hart, first. I think Briggs is probably too much of a tank for either Hart or Spinks to handle.
I've never seen Hart fight. But he did beat Jack Johnson, although some disputed that decision I think he probably gave Jack a close hard fight at least. I've seen Jack Johnson. I think he was good. Leon Spinks was a good novice. His achievement in winning the title in his 8th pro fight against Muhammad Ali is severely underrated. Spinks was decent for a couple of years but never developed as a fighter. Briggs was crap. His claim to fame is being gifted a decision against a 48 year old. His other win was against Liakhovich, a forgettable Russian who had one good win in total. I'd expect Spinks to beat Briggs. I can't assess how Hart would fare against them.
He was a martian from outer space that the boxing commission kept on the down low by giving him a plumbers job.of course this was before Roswell with the other lifeforms. They would also have given him pectoral implants had they existed back in the day..that was a dead give away. His enlarged liver which he died in his 50's from was the result of his bigger red blood cells that required more oxygen. Had he been on his own planet he would have been around 4' 2.
All that we really know about Hart is is general style, and roughly how big he was. His resume leaves a lot of room for interpretation, regarding how good he was head to head. He had a fast rise, a short prime that included a lineal title win, and a win over an all time great, and an equally sharp descent. If he had finesse in terms of head movement and footwork, then his chances against Briggs would look a lot better. Basically we don't have a clear idea of what we are sending in against these guys.