the orig.amt willys kit from AMT, consisted of a willys gasser that could be built as a coupe or pickup with a 394 ci blown olds. as this was a 2 car kit, the second car was the 32 ford sedan body with a tri-carb buick "nailhead" engine.
The "nailhead" buick was introduced in 1953 for buicks 50th anniversery, at 322 ci, in 1957 it was enlarged to 364 ci, in 1959 it was enlarged again to 401 ci, starting in 1964 you could get the 401 ci or an enlarged version at 425 ci. this lasted thru 66. in 67 a new family of engines was introduced.
the v6 at 225 ci was not sold to jeep till 1967.
hope this helps
Al