Hi. The two pumps are interchangable, if the spacer is used with the later pump.
377-400 will fit on all the engines without a spacer
377-410 will fit on all the engines with the 377-415 spacer.
The difference between the pumps is the length of the arm. That spacer and longer arm pump were introduced in 1978 to reduce heat problems. The longer arm pump and spacer can be retrofitted to the early cars.
Hope this answers your question.
Kelvin Dodd
Global Sourcing Engineer
Moss Motors, Ltd.
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