Seems, the questions are: 1.) What was marketing and what was engineering? There is a big difference from the "stated" 265 to 320 HP in 68' and 265 to 325 HP in 69'. Different heads, valves, intakes, carbs, cams, suspension, and dual exhaust. Seems like a good choice for the reported $180.58 (350 HO) and $76.88 (400 HO) pricing.
2.) Did marketing really drive all those options/differences (with different PNs) or did engineering dictate them for performance improvements?
It's amazing how advanced both marketing and engineering were in those days and how many options/upgrades (see below) were available.
1968: Sales climbed 23 percent to 107,112 units, with 90,152 Hardtops and 16,960 Convertibles. Trim was divided between Standard and Deluxe models. Buyers chose 15,969 Standard six-cylinder cars and 2525 Deluxe models, and 55,882 Standard V-8s and 32,736 Deluxe ones. Base prices were $2781 for the hardtop and $2996 for the convertible. For another $273.83 buyers could get the 330-bhp 400 model with twin scoop hood, heavy duty suspension, and red line tires, and the 335 bhp 400 HO cost $350.72. Top dog was the 400 Ram Air package for an additional $616, which raised the engine’s peak rpm to 5,200 but maintained 335 bhp.
As in 1967 the options were numerous and could add $1000 to the cost of the car. Common add-ons included air-conditioning ($360.20), power brakes ($42), front disc brakes ($63.19), power steering ($69), power windows ($100.05), power top ($52.66), vinyl roof ($84.26), fold-down rear seat ($42.13), hood tachometer ($63.19), tilt wheel ($42.13), console with bucket seats and floor shift ($47.39), cruise control ($52.66), head rests ($42), remote control trunk lid ($13.69), rally gauge cluster ($84), reclining right hand seat ($84), AM/FM radio ($134), stereo tape player ($128), tilt steering ($42), 3-speed with floor shift ($42), 3-speed synchromesh ($84), close-ratio 4-speed manual ($184.31), automatic transmission with V8 ($236.97), wire wheel discs ($52.66), Rally 1 wheels ($40), Rally II wheels ($56), two-tone paint ($114.80), power bench front seat or LH bucket seat ($69.51), adjustable front and rear shocks ($52.66). https://www.hagerty.com/valuation-tools/pontiac/firebird/1968/1968-pontiac-firebird
