[1966C30Stakebed]

Here is what I found on the web.

300[edit]
In 1964, Buick replaced the 215 with an iron-block engine of very similar architecture. The new engine had a bore of 3.750 in (95.3 mm) and a stroke of 3.40 in (86 mm) for a displacement of 300.4 cubic inches (4,923 cc). It retained the aluminum cylinder heads, intake manifold, and accessories of the 215 for a dry weight of 405 lb (184 kg). The 300 was offered in two-barrel form, with 9.0:1 compression, making 210 hp (160 kW) @ 4600 rpm and 310 lb·ft (420 N·m) @ 2400 rpm, and four-barrel form, with 11.0:1 compression, making 250 hp (190 kW) @ 4800 rpm and 335 lb·ft (454 N·m) @ 3000 rpm.
For 1965, the 300 switched to cast-iron heads, raising dry weight to 467 lb (212 kg), still quite light for a V8 engine of its era. The four-barrel option was cancelled for 1966, and the 300 was replaced entirely by the 350 in 1968.
The Apollo 5000 GT sports car, (also sold as the Vetta Ventura) used this engine.
340[edit]
The 340 cu in (5.6 L) 340 was a stroked to 3.85 in (98 mm)) version of the 300. It had a two- or four-barrel carburetor, the two-barrel with 9:1 compression ratio rated at 220 hp (160 kW) at 4000 rpm and 340 lb·ft (460 N·m) at 2400 rpm, and the four barrel with 10.25:1 compression ratio, rated at 260 hp (190 kW) @ 4000 rpm and 365 lb·ft (495 N·m) @ 2800 rpm. It replaced the four-barrel 300 for 1966. It was produced only in 1966 and 1967, with the new Buick 350 taking its place after that.
350[edit]
Buick adopted the popular 350 cu in (5.7 L) size with their final family of V8 engines. Although the Buick design shared the displacement of the Chevrolet Small-Block engine family, the Buick blocks were substantially different and of Buick's own proprietary design.