What engine does your 57 have in it now? If a V8 then yes you can mount that bellhousing to a 70 250, but if the 57 has a 235 6-cyl then it will not...
There is a good tech tip on this site about the late-6 bellhousings
here . The 55.2 and later trucks are better candidates for a 230-250-292 swap than the prior years. The flywheels are different too. Good luck!!
