Your original 51 chev trans/torque tube drive line will NOT bolt up to any other bellhousing than your stock one. I assume you are talking passenger car not truck. The bellhousing to trans pattern for passenger cars was a real odd ball from 40s to 54, 55 passenger bellhousing went to the 'modern' trans pattern. My old chevs range from stock, torque tube to resto/open drive and are 40/50s. I can say now that I've been at this for a while, switching over to open drive line and more modern trans should have been first on my list.
The 230/250/292 motors are a great swap for your car, even though you'll need to upgrade to the correct SIDE motor mount kit. I'd check with Tom Langdon at Langdon's Stovebolt(his ad running on the home page), his online catalog should show you the mounts you'll need.
Fan clearance is a concern but a minor one with several solutions, from moving the radiator to eliminating the stock fan for electic unit. With this modern six you'll get a bonus as ANY modern trans will bolt right up, no adaptors needed.
