1934 Series 40 all came with the small st eight, This engine was new for 34 and became the basis for all small Buick eights.
The 320 roadmaster engine came out in 1936 and was perity much unchanged untill the last in 1952. Engine mounts changed and rods became inseret.
If the engine I.D. pad is located between the distributor and the firewall, it is the big motor.