Here are two typical examples of a big block and small block engine. Typically, .10, .12, .15, .18, and .20 small block engines use a smaller crankcase. Typical big blocks (.21, .26, .28, .32, etc.) use a larger block which is longer and wider than a Smallblock. So in most cases, a Smallblock to big block conversion requires a different mount and usually a different flywheel and clutchbell. Pictured is a Traxxas 2.5R and Novarossi BX21 (Ron Paris modified).

In some rare cases, a Smallblock mount can be modified to fit a big block, but mounting the engine is one thing. Accounting for the longer case and crank and throttle linkages is a different thing. Only a rare few manufacturers make a .21 displacement in a Smallblock case. Typically these engines are over-bored and generally won't run the same rpm and make the same power as a larger crankcase engine will.