The biggest problem will be the position of the water pump. In mid year 1955 the pump was moved about 3 inches lower on the 235 block and a different shaft and pulley arrangement was used. To use the later engine in the 1951 truck you'll need to install a water pump with a shortened shaft, or use a relocation plate a member here sells to install the 216/early 235 water pump. Just about everything else is a bolt-in swap. Be sure to use the flywheel and starter from your 216 engine to avoid a mismatch between the 12 volt 55 starter and the 6 volt flywheel. The tooth pattern from 6V to 12V is different but the 6V starter works well on 12 volts if the truck has been converted to the high voltage system.

Buy a large bottle of Advil or some other good headache remedy- - - - -you'll need it when dealing with the dual carbs!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!