Can anyone tell me if a new model blackhawk in .357 made in 1974 is the same frame size as an old model .357?? (s/n starts in the 32 range)

No, they're not the same. The New Model 357 cylinder and frame is essentially the same size as the flat top and old model 44.

So would an old model .357 be smaller than a new model .357 or a flat top??

Yes, an a new model 357 Blackhawk frame and cylinder is larger than an old model or flat top.