your question requires more information to accurately answer..... and the answer is "maybe"
year matters... if you have a "late" cobra, say 1995 or newer, its actually closer to a Volvo SX, look at those for a "new" drive.... if its earlier, then you can proceed to second question, as IIRC only the "early" cobras had the 2.3 Pinto motor....
what gear ratio do you have with the 4.3, and what was offered with the 2.3.... IIRC they are not the same ratio, so you will need to try and re-prop to compensate, if there isnt a huge difference in ratios....