are 170ci blocks and200ci blocks the same

AFAIK there were no changes made between those engine blocks until after 1980, when the bellhousing was redesigned on the 200. But you won't know for sure until you get them beside one another, the tranny length will probably play a bigger factor in getting it in there than anything else, and potential modifications in driveshaft length. Change it as a combo, engine and tranny.

You can also get a 250. This article is helpful -- https://en.wikipedia.org/wiki/Ford_Straight-6_engine#250

the 200 is more hot rod able and easier to get parts for since its based on the 60s mustang 200 still .. better intakes chrome kits and internal aftermarket parts.