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actually, 400 and 800 series uppers are the same -exact same housing, but again it's the gears inside that will make it work properly with your engine's power. 400 series lower units are not compatible with V8 power, but the uppers are.i know that per say a 400 unit will not fit on my 800
the SELOC manual for OMC sterndrives 1964-1985 has a good reference in it with all gear ratios, upper, lower and overall.Or even a reference of what i can look at to find out if one of the ones on ebay might fit mine via years and or HP ratings that might be the same ratio as mine.