A 65 273 4 speed should be a small retainer IIRC.

The 273 shares the same bellhousing bolt pattern as the 360 so if the 273 flywheel is the same diameter as a later 360 LA (130 tooth) you only need the flywheel.

Or you could have the 273 flywheel rebalanced to be a 360 flywheel (as long as the bolt pattern is the same) Getting past 67 and things are a little different and fuzzy for me.

If you get a new bellhousing and the hole is larger then you can make/buy an adapter ring. I did on my /6 4 speed.

Brewers or Passon can sell you exactly what you need if you don't have it.