1976 mercuiser 233 351w outdrive question

[MAILMAN44094]

From what I tell from the outdrive identification photos I have a alpha upper unit (flat top, smooth sides) with a pre alpha lower unit (round cavitation plate). What impeller kit do I have to order, alpha or pre alpha. I can't find any serial numbers on outdrive.
TIM

 

[haulnazz15]

Re: 1976 mercuiser 233 351w outdrive question

It would be difficult to say without pictures and likely the ratio stamped somewhere. The 233 drive was a 1.32 ratio, but if it has been swapped out, it's anybody's guess. Probably a 1.5 ratio if they went to an alpha upper. Prop specs and wot RPMs/top speed may also narrow it down.