Does anyone know any prefix or something to help me please. I need ot know how to be sure that a 351 is a 351 and not a 302, other than the colour of the rocket covers. I am taslking a clevland from the 70's and early 80's. I got stung once before and i didnt find out until years later i had actually got a 302 not a 351. Was a nice donk though

Measure the stroke with a piece of wire down the spark plug hole.

As pete has indicated.
Take a plug out. Any one will do. Put a piece of stiff wire down plug hole and rest it on the piston.
Wind motor over by hand til the piston gets to it's lowest point. Mark with a texter. Then wind motor again til piston gets to it's highest point. Mark it again.

Now take wire out and measure. From one mark to the other.
3" = 302.
3.5" = 351.

Gee, thats nice and simple isnt it. I love simple logic like that
Thanks chaps