Most likely a 350 cubic inch or 400 cubic inch engine
Pontiacs did not have "big" or "small" blocks. Your '68 could have come with a 350 or a 400 cubic inch motor.
2006 Arctic Cat 400 TRV Auto engine size in cubic inch?
Motorcraft.com does not list a liter size for the 390 - they only refer to it as the 390 cubic inch . ( the 400 cubic inch is the 6.6 L / the 360 cubic inch is the 5.9 L )
a Ford 400 had a 4 inch bore and 4 inch stroke. Simplest math motor EVER!! Given 4" bore x 4" stroke x pi (3.14) x 8 cylinders = 402 cubic inches. Bore this thing out .060 and you get: 4.060 x 4 x 3.14 x 8 = 408 cubic inches.
( The 4.9 liter / 300 cubic inch inline 6 cylinder ) ( 5.0 liter / 302 cubic inch V8 ) ( 5.8 liter / 351 cubic inch V8 ) ( 6.6 liter / 400 cubic inch V8 ) ( 7.5 liter / 460 cubic inch V8 )
www . motorcraft . com ( no spaces ) shows : For a 1972 full size Ford with 400 cubic inch V8 engine , a spark plug gap of ( .035 inch )
388
6.55 liters
A 400 cubic inch engine displaces about 6,554.8 cc
Orig Answer: Should be around 409 Actually the conversion is 1 litre to 61.024 (rounded) cubic inches thus 6.2 x 61.024 = 378.35 (rounded) so the 6.2 litre would be 378 cubic inches 6.6 is 400 cubic inches
255bhp.