If I remember the 64 was the last year of the course spline (10??) output shaft and the 65 went to the fine spline. This would make a 55-64 a direct bolt in. The standard OD rear end was a 4.11 and if you have 3.73 gears then the final drive would be 2.61 in OD which might be a bit shallow for hills or head winds when behind a 230. I sure would give it a try because these units can be found at nearly every swap meet that is of any size and usually around $150. Try to make sure the solenoid is good since if not a good one can run as much as the whole trans cost. In addition to the lockout cable and wiring mentioned you might have to fab up different shift rods unless the shift cover off your old trans will bolt onto the new one or they're already identical--I'm never that lucky.

Evan