I am working on a 251 to replace the stuck 230 in my 52 dodge pickup. So far so good.
When I got the 251, it didn't come with a starter so I put the 230 starter on it. This is before I realized the 251 could have a different flywheel tooth count. The engines that you have in front of you should all have 146 tooth ring gears. The 172 tooth ring did not appear until 1957.
Anyway putting the 230 starter on was no problem and it worked fine - didn't bind up and sounded ok. So is this normal? Yes.
First - could the 230 flywheel fit on this motor (explaining why the starter didn't get bound up)? Yes.
The 52 has a three speed fluid drive. Will I be able to bolt the fluid drive to the 251? Yes.
As previously mentioned, there is a difference with the 201 cranks and flywheels in terms of 'depth', but there is also a difference in the number of bolts, also previously mentioned.
All of the fluid couplers and torque converters will have and require 8 bolt attachments. There are 4-bolt and 6-bolt cranks/flywheels in some of the older 23" engines but not in the 25" engines.
...clear as mud, eh?