So, here's my best effort to summarize what we know (and what we don't) about the development of the Boeing NLA (and the related VLCT & LAPD programs). They all appear to use the 763- prefix to their design numbers, which is one of the series Boeing uses, this one for large aircraft.
763-001 to 763-017: No details known
763-018: Double-decker NLA with circular (334”) cross-section. 776 pax (2-4-4-2 main deck, 2-4-2 upper). 260’2” long (fuselage 250’6”), 275’8” span, ~77’ height (approx.), 4 x GE90-class engines
763-019 to 763-031: No details known
763-032: BWB-delta design NLA. 609 pax. No other statistics. 3 x turbofan engines
763-033 to 763-057: No details known
763-058: Double-decker NLA. 650? pax (presumably 2-4-4-2 main deck, 2-3-2 upper). 232’2” long (approx., fuselage 214’4”), 281’2” span, 81’4” height. 4 x GE90 / Trent ??? / PW4073 engines.
763-059: Double-decker NLA. 757 pax (2-4-4-2 main deck, 2-3-2 upper). (fuselage 244’4”), 281’2” span. 4 x P&W STS977 engines.
763-060 to 763-164: No details known
763-165: Single-deck horizontal double-bubble NLA w/ shoulder wing and V-tail. ??? pax. 254’3” long (fuselage 227’5”), 260’ span, height ??. 4 x turbofan engines.
763-166 to 763-188: No details known
763-189A: Early 747-600X study. 503 pax. 220” fwd & 122” aft stretches. New wing. 747SP vertical stabilizer. New engines.
763-190 to 763-222: No details known
763-223: Late 747-600X study. 548 pax. 277’5” length (270’ fuselage), 255’ span, 71’7” height. Year: 1996.
763-224: Late 747-500X study. 462 (?) pax. 253’7” length (245’ fuselage), 255’ span, 73’8” height. Year: 1996.
763-225 to 763-240: No details known
763-241: Single-deck LAPD w/ shoulder wing and T-tail. ??? pax. 250’ long, 262’ span, 66’ height. 4 x turbofan engines. 27’ wide passenger cabin. Year: 1999.
763-242 to 763-245: No details known
763-246C: Single-deck ovoid LAPD. 454 pax (2-4-4-2). 240’9” long (fuselage 231’8”), 262’ span, 75’11” height. 4 x 66,800 lbf turbofan engines (104” fan). Range 7,750 nmi (9,000+ nmi for -246CER version). 58 overhead bunk beds. Year: 1999.
763-246CS: Stretched version of 763-246C. 547 pax. 273’5” long (fuselage 264’4”), other dimensions same. 74,500 lbf turbofan engines. 82 overhead bunk beds. Year: 1999.
763-247 or greater: No details known. Last 763-prefix design number unknown
NLA designs whose 763-xxx numbers are unknown:
Double-deck NLA with pinched upper deck (ovoid figure-eight). 606 pax (2-3-4-2 main decker, 2-4-2 upper). 244’2” length (approx., fuselage 229’3”), 260’ span, 77’8” height. 4 x GE90-class turbofan engines. Range 7,850 nmi. (Corresponds to in-flight art posted in this thread. Also matches interior drawing labeled PA-797-4I-6 R2 C). Year: 1994.
Stretch of the above to 750 pax. (91 m [298’6”] long, said “Flight International.”
Interior drawing label PA-797-3I-5 R2 C: Single-deck NLA. 485 pax (2-4-4-2).
1 of 5 reported in “Flight International” article of 1993-06-15: “Benchmark” double-deck NLA. 624 pax (18-abreast over two decks). 249’9” [76.13 m] length, 272’4” [83 m] span for 14,500 km range OR 259’2” [79 m] span for 12,000 km range, 4 x 95,000 lbf turbofans.
NLA design, 763-number not known, 2 of 5 reported in “Flight International” article of 1993-06-15: Shortened double-deck NLA. 584 pax (18-abreast over two decks). 234’9” [71.54 m] length, no other notes.
NLA design, 763-number not known, 3 of 5 reported in “Flight International” article of 1993-06-15: Single-deck NLA. 627 pax (15-abreast over two decks). 244’4” [74.49 m] length, said to resemble a wider 777 stretch. 4 x turbofan engines.
NLA design, 763-number not known, 4 of 5 reported in “Flight International” article of 1993-06-15: “747-look” 1.5-deck NLA. 630 pax. 242’9” [74 m] length. Some features related to the 777, but with an upper forward deck.
NLA design, 763-number not known, 5 of 4 reported in “Flight International” article of 1993-06-15: A stretched version of the benchmark double-decker could seat 750 passenger in 3-class seating.
NLA design, 763-number not known, from “Aviation” magazine 3-view [French]: Double-decker NLA. 234’11” [71.6 m] length, 257’10” [78.6 m] span, 77’1” [23.5 m] height, 27’10” [8.48 m] fuselage cross-section. 4 x turbofan engines.
Other aircraft presumed to have a 763-prefix number:
(a) The VLCT designed in cooperation with Airbus, with capacity up to 800 passengers.
(b) The 747-700X, if it ever progressed to the formal drawings stage.
That's all I know. It seems like there's a lot of "big Boeing" history still waiting to be learned.