273 crank in 318 bennifts??

Part of the 340s success is its lighter rotating assembly. A forged steel crank can be used in a 318 or 340 if properly balanced.
As a general rule of thumb all engines painted Chry' industrial red have forged steel cranks. This isn't carved in stone though. Some orange and blue engines has forged steel cranks too.
As far as I know , every 273 ever built and all 1968 model 318s had forged steel cranks.
The only sure fire way of knowing which you have is go in and look.
A similar mystery exists in the bowtie world too. Does their 350 have 2 bolt main caps or 4 bolt main caps ? Only visual inspection will answer that question.