I can't answer that...
But I'm sure you can find an overdrive transmission. These were fitted to cars right up to 1960 or so, though the input shafts were longer (which might be a bonus if that fluid drive takes up much room) about '57 to '60.
One guy in Vegas has several, though he probably wants to keep them, but I don't think he's looking for more and therefore might be able to put you on the trail of one.
As will others here.
Alternatively, I'd reckon you could just about fit an A833 overdrive 4-speed (alloy housing A833) without too much trouble, and they're fairly easy to get and fairly inexpensive.
I don't think you'd have a 251, by the way. 245, maybe, but more likely a 218.
Measure the length of the head. If it's under 2' it will be a 201 or 218, if it's over 2' long it may be bigger.
