B20 should pretty much bolt in with no problems. You'll need increased fuel -- if you have the original SU carbs with ZH needles in them (usual for the B18D that's native to your '65 if it's a 122S), you'll need to go to SM needles switching to a B20B (dual carb B20) or KD needles for a B20E ('70/'71 high-compression EFI motor) or B20F ('72-'75 low compression EFI or CIS), all assuming the original cams and heads are used. If you have a Weber downdraft carb conversion, you'll need to rejet it.

Easiest way to convert an injected motor to carbs is to remove the injectors and cap off their holes with suitable freeze plugs. This permits reverting to injection later, if you eventually decide to use that motor elsewhere or fit injection on your car (which is a neat thing to do, performance-wise).

Best of luck,

--Phil