Yes, that.
Jon, it's not a big deal to fit a later engine into an earlier chassis, if that's what you want to do; but as you have surmised there are a few snags to overcome. I honestly can't give you a blow-by-blow as mine was done decades ago, and by someone else at that, and even then on a car which was being built around the engine. However if your intention is merely to use the 3-brg as a spare or temporary replacement, these sound to me like problems you could live without. Would it not be a better idea to acquire a spare engine which is directly interchangeable with the one you have? Then in times of need it is a simple matter to exchange the two.
Jon, it's not a big deal to fit a later engine into an earlier chassis, if that's what you want to do; but as you have surmised there are a few snags to overcome. I honestly can't give you a blow-by-blow as mine was done decades ago, and by someone else at that, and even then on a car which was being built around the engine. However if your intention is merely to use the 3-brg as a spare or temporary replacement, these sound to me like problems you could live without. Would it not be a better idea to acquire a spare engine which is directly interchangeable with the one you have? Then in times of need it is a simple matter to exchange the two.