Is there any way to easily tell these apart before I pull the engine from the truck?
The engine I am looking at ha a ridge on the edge of the flat area where distributor goes in.
Thanks,

 

If you are 100% sure of the originality of the motor and the valve covers, the sticker on the drivers side cover will tell the ci displacement. If not, the only method is to check the crank stroke. 400= 4" 351= 3.5"

Nathan

 

The fastest way to tell is to take out a sparkplug, make sure the piston is at top dead center, and put a long pencil on the plug whole. Spin the engine till the piston bottoms out and measure on the pencil how far it traveled. As mentioned above, 3.5" = 351 and 4.0" = 400.