Ypu measure from the inside top edge of the hook on the transom clamp to the cavitation plate above the prop. That is where your measurement is. If it was a 20" shaft there would be an extension housing on it.
Here is the one I bought for mine it comes with great instructions and is about a 1/4 of the price of the CDI one.
https://www.ebay.com/itm/174785668696?hash=item28b209a658:g:bEwAAOxyn9BRaadt
how much weight do you think you would actually save? The Verado set up at 870#s doesn't include the extra bracing you need for an Outboard conversion. Not to mention the repairs an a small block Chevy will be significantly cheaper than the Verado. Not to mention the small block will be simpler...
one other thing to ask the owner is the oil setting. That determines which oil to use. If I was buying it I would take it to a dealer and have them set the pump as rich as possible. then run the best oil you can find XD 100