Here's what I have found playing with these 273's for over 40 years now, and not counting exhaust manifolds, oil pans, motor mounts, and other bolt-on parts. The '64 only came in a 2bbl version, and had the 315 casting heads with matching intake. Forged crank, solid lifter cam with adjustable rockers. '65's were pretty much the same except that they added a 4bbl version that came with a higher compression piston and higher lift cam. '66 started (for the most part) the 920 casting head and matching intake....in both a 2bbl and 4bbl version. They also came in both a C.A.P. (Cleaner Air Package) and non C.A.P. '67 continued that same engine. In '68 and '69 there was only a 2bbl version, 675 casting heads (same as the 318 those years) hydraulic cam, and a cast crank with the larger convertor register (also same as the 318). The blocks also have a different casting number. The only thing I remember being different on the block is the number of motor mount bosses. The '64-7 have 3 bosses on each side, and the '68-9 have all 4 in most cases. There are a few oddballs though. I've seen a couple with 3 on one side and 4 on the other. Very weird. some of the early 318's did the same thing, but, when they had 3 bosses, it was a different 3 than the 273. This required a tab to be added (welded) to the motor mount bracket to get all 3 bracket bolts to be used. In your case, any of the those 273's or early 318's will bolt into your car by using your existing 273 bolt-on parts. The only exception here is if your B-body has a 727 transmission behind it. In that case, you would need an engine with the large convertor hole, or, you would need to swap in a crank with one. And yes, before anyone argues about it, they did make that option in a B-body. Very rare, but, taxi's, police, and special orders had them (not to mention and the poly 318 cars that year). Those vehicles used a 318 forged crank like what also came in an A100 van with a 273. If you want some cheap horsepower, I'd get that 318 engine as a spare to build. Put it in when you want, run it as hard as you want, and keep the original engine for when you want to start a restoration project on the car.