Oil leak on rear of engine

[classic car repair]

I have a 1938 Ford Pickup 1/2 ton, flat head v8. When it in the shop running, no oil leak, If you run down the road about 2 miles and back, put in the shop, there is a large oil leak on the floor, coming out of the bell housing, can it be a rear main seal or a gasket on the back of the engine? It only leak bad after a road test, not running in the Shop? Thank You