I'm almost positive the 273 heads are CC so as BBcuda said you'd be worse off. what about locating a 318 or better yet a 360? (& the 360 would only require a slight mtr mt adjustment for fitment & the B&M 10239 flexplate), small price to pay for the very significant gain you would experience (especially with the 360)
Bit more than that, early 273's have a slightly different driver's side configuration and the old 1/2" spacer on a 360 trick doesn't fit it all, you need to weld an ear on the MM as well. See pic. Schumacher Creative has the swap mount for those of use that cannot weld (me). My 65 273 is meh in the power department, so I upgraded it to an 87 318.
