Keep in mind that your early 273 has a smaller snout on the converter. Starting in '66, I believe, 273, 318, 340 & 360 engines had a bigger register in the crank and corresponding bigger snout on their converters. Unfortunately, it's not a simple converter swap because the input shaft splines are different between the small and big snout converters, so you'll need a later trans also. (Unless you get yours rebuilt by a knowledgeable Torqueflite mechanic who knows what has to be changed out/modified so you can keep everything else as-is.)
When looking at later transmissions, late '70's started using lock-up converters. Those converters don't interchange with earlier versions as well because of the splines and lock up features that extend to the valve body also.
Your trans is a 904.
With a bigger engine, it might be worth considering upgrading your brakes, for when you might want to "enjoy" the extra HP!
