I doubt that it is that easy. Nuclear thermal engine research also took place in the 1980s and 1990s, all with promising results in terms of feasibility but still with MANY unsolved problems to actually build and operate one such engine.
How many reactors do you know, that can operate at the chamber temperature of a SSME? While getting rid of the oxygen in the exhaust means 3 times the specific impulse alone, reducing the temperature in the chamber to what nuclear fuel elements for existing high temperature reactors can take also reduces it again, resulting in a only slightly higher performance in the end.
No, the problem of building an NTR with significantly better performance than hydrolox *was solved* in the 1960s.
The NERVA project built engines that were actually fired on the test stand, all of which performed at over 800 seconds vacuum ISP (nearly *twice* that of the SSME) and gigawatt power levels, for thrust on the order of 250 kN, with chamber temperatures on the order of 2200 K. One was fired at full power for 15 minutes, and then restarted for another 14, another did a burn lasting a full hour, and the final one tested was deemed ready for spacecraft integration. In the end, Nixon killed the project despite a specific stipulation by Congress in that year's appropriation for NASA that funds were not to be diverted away from NERVA.
[Rant]I was aware NERVA hadn't actually flown, but what Nixon did to it I only just now read while gathering exact details. The
killed a flight-ready NTR, and abandoned Apollo/Saturn in favor of the Shuttle boondoggle. We've only just now regained a man-ratable booster/capsule (the Shuttle may have gotten the "man-rated" stamp, but it certainly never *should* have), and a replacement flight-ready NTR is still a ways off, if it ever happens. Nixon set the US space program back at *least* half a century.[/rant]
The more recent 80s and 90s project you refer to (Timberwind) was, AFAIK, a paper design, which, if feasible, as you say, would have offered ISP in the 1000 second range.
The 2019 federal budget funds development of an NTR with plans to flight test it by 2024. I'm not horribly optimistic about that, between the likelihood of a change of administration in November and the fact that it would be starting from scratch, but if they kept their goals modest (design conservatively rather than bleeding-edge, don't try to get better performance than NERVA) it might well be doable. But I figure it's fairly likely that they'll try to match or exceed Timberwind, in which case they'll be going into uncharted territory. I *know* 800 seconds is 60s tech. Since I haven't heard of any Timberwind engines being built or fired, I have no idea if 1000 is 80s tech, or 2000s tech, or 2020s tech, or 2080s tech.