200 cid head on a 250

I have a 69 coupe that the door tag says the engine is a 250. It also has the lower positioned starter that ensures me it is the 250. It runs but I can't get out of my own way. I have a rescue car I'm building in the back yard that I thought had another 250 in it but turned out to have a 200 in it, as evidenced by the starter above the oil pan seal. Now I got to snooping around with the casting numbers on the intake and it seems I have a 200 head on the 250 block in the mustang we have been driving around. which would explain the lack of bottom end power. Both casting numbers are C9DE-6090 The rescue car number is followed by a J and the driver manifold has a -M as the last position. Am I right are both of the heads off of a 200? and wouldn't it have been smarter to put a 250 head on instead of the 200 head it has on it. Thanks:shrug
JimP