The raw math is
gpm x 60minutes x 8.34lbs/gallon x F-rise= BTU/hour requirements.
In a shower, figure on ~105F. If you assume a min incoming temp of 40F (probably true for most of central/western MA- for me it's a bit above your measured 42, ,but not by much.)
If you're assuming 6.5gpm you have a net requirement of 211,419BTU/hour, which you AIN'T gonna get out of any tankless. The NR98s output will be ~45KBTU/hr shy of filling the bill. A condensing tankless with a 199K burner would still be 15-20K short.
A 75gallon tank with a 75K/80% burner would probably make it.
The other solution would be to install a drainwater heat recovery heat exchanger (PowerPipe or similar) big enough to return 50%+ of the heat going down the drain measured at the 2.5gpm standard. At 6gpm it'll still be delivering 40% back. This would use substantially less fuel too. A list of models tested by 3rd parties is maintained by Natural Resources Canada here:
http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/drain.cfm?attr=4
You'll need at least 5' of vertical drain to fit one in, and both length & diameter increase the heat-transfer efficiency. The output of the HX need to feed both the water heater & shower's cold feed (but if it has to be one or the other, you'll still get ~75-80% of the performance feeding just the hot water heater.)
At 2.5gpm it's like having another 30K of burner behind it, but at 6.5gpm it'll be more like an additional 80-90KBTU/h. With that a tankless solution would have some margin, and you might even squeak by with a high-recovery rate 50 gallon tank like the Bradford-White M-2-XR504T6FBN. Figure on a ~$1-1.5K installed cost for drainwater heat recovery. EFI in Westboro MA reps PowerPipe in the US, and have been known to sell at the wholesale price in onesie-twosies (it doesn't take much to set up an account with them either.) It's not necessarily about payback on fuel savings- think about the price/performance of alternate hot water heating systems that would actually support the load.
Last, and definitely not least, if you're heating your house with a boiler that has more than 150KBTU/hour of output, and indirect-fired 50 gallon tank running as a priority zone off the boiler would fill the bill for less cash outlay. Even if the boiler's output is ~75KBTU output and indirect tank + drainwater heat recovery or just an 80gallon indirect would still come in at about the same or lower installed cost (and lower operational cost) than a tankless that's still falling short, or a 75gallon/75KBTU standalone that's just squeaking it.