Your '71 E should have a 6 bolt engine, unless the engine was swapped at some point.

It is possible that your '71 E had an F head put on at some point. Hi octane gas did a sort of dip for a while, and it got very hard to drive the 10.5:1 B20E's around without doing something to lower the compression. Thick headgaskets were one way, later model F heads were a better way. Perhaps someone did that to your engine and kept the 'E' head? Either way, I don't think you'd get 30 more HP from putting a stock E head on in place of an F head. Maybe it's been worked upon? In any case, gasoline quality has come back somewhat since then, and you can again use a 10.5:1 compression head if you like (and you don't mind paying extra for premium). You can measure the thickness of the head from the bottom face to the top machined face to see what type it is.

If your 'spare' motor is a B20 it will have a fairly prominent cast 'B20' on the block. Right there underneath the #1 sparkplug area more or less. Hard to miss if it is there. If it isn't, it is a B18 - nearly identical engine with smaller pistons (and valves).

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