Are all 273 V8's the same?

So just to sort some of this out - you asked if the whole engine is a bolt in and the answer is yes since you would use the exhaust manifolds and engine mounts from your 65 B-body (which you already have). Because of the difference in heads and intake due to the switch in intake bolt size and angle between 65 and 66, you wouldn't want to interchange these parts between the 2 engines in the swap unless you moved heads and intake together, but as long as you are using all the engine parts on one motor or the other, yes, just drop it in...