Attached is a Volvo Penta schematic showing that your 3.0 1999 engine uses a riser 3857798,
The attachment showing 3857798 shows the Volvo Penta system page showing that 3857798 has been superceeded to a 3863071. This is the riser that Volvo Penta believes you should be using.
The attachment labeled VPA expired 3862603 shows the Volvo Pents system showing that the 3862603 number has been replaced by a kit. The kit part number was 3588684.
Part number 3588684 has been superseded by 3857798 which in turn has been superseded to 3863071. Please see the file supercesshistory 3858684 to see this.
So everything eventually supersedes to 3863071 (which comes with a bushing and a plug by the way).
I show that the 3862603 part number was originally on a 1993-1994 era Volvo Penta engine. The last time we sold one of these was in 8/16/2010. If I had the part in our stock I would be happy to sell it to you for the $78.62.
I have looked around everywhere for anybody who has one of these parts in their shelves I would get it for you. If you can find anyone out there in the world that has this part, please let me know as I would like to purchase some for our stock.
I also found that BARR makes an aftermarket version of the original number 20-3862603, but again I couldn't find anyone who has one.
I am sorry I can't fine the 3862603. But please remember that Volvo Penta says that this is not the correct riser for your engine.
If you would like to discuss this further please feel free to contact me directly.
J. D. Neeson