the Barris site list this for the engines "Dual 340 cubic inch 1966 Buick engines".
something from a wiki:
340
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 compression of 9 to 1 comp. 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 to 1 comp 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.