Small block to Big block swap.
 

I have a Volvo 5.7GI and was thinking about doing a complete engine swap with an 8.1GXI. The drive is a Volvo SX-M 1.43 ratio. I know the 230LX stingray came with a big block for a few years then Stingray dropped the BB option. The hull is the same as the 230SX hat has both Merc and Volvo BB options. The BB powered 230LX is like a unicorn to find used so I opted for my 05 with the SX drive as the Mercs commonly came with the A1 mated to the 5.7. So, what are the odds I can drop a complete 8.1 Volvo and have it bolt right up to my SX drive? I’ve made a few calls to some performance shops and they really don’t know other than said just swap the ECM to a 5.7 GXI for added HP. I guess I could try it but I’m used to a BB powered boat.

I had a 230SX Stingray with a 496 Gen 7 BBC. I sold it after a couple years of ownership. At one point Stingray moved the rear seat forward on the 230LX so it would accomodate the longer length of the BBC engine. I don't remember what year they did that, or if small block boats came with the rear seat further back and the big block boats had them further forward on the same model year. The BBC On my 230 SX the pulleys were very close to the back seat, and you could just get the belt through that space to get it on. I don't know enough about Volvo drives to answer that question, but if it had a sbc merc with an Alpha 1 drive, you would have to change the drive to a Bravo drive to handle the power of a 496BBC. The Volvo used the same GM engines as Mercruiser did, so the engine dimensions would be identical. A 454 or 502 BBC is about 2" longer all in front of the motor mounts. I am not sure if the 8.1 BBC is also identical to that but I believe it is.

I’ll have to measure the seat to transom opening, it looks like about 3-4 inches between the seat back and the accessories at the closest point.

If you have 3 - 4 inches between the accesories and seat back and the same between the harmonic balancer and floor, I think it will work. I standard BBC like a 502 would work for sure since they are slightly less than 2" longer than a SBC. I would have to get the actual length measurements of a Gen 7 496 block to be certain.