The first thing that you will need to find out is if the engin mounts are the same and as you are going from a 4 cylinder to a 6 cylinder I would have thought that they would not be the same.

Unless you are desperate to keep a volvo engine in your current beast then the alternative might be to go for a chevy v8 350 or similar. I am told relyably that the brakes are more than capable of stoping that much power and there are remounting kits available (in Sweden at least) for such engines. It will mean a lot of electrics work but so will the conversion from 4cl 6cl even if you stay with the volvo. The other problem with changing the engine is that you will need a new ECU although these can be obtained from a scrap yard, where they are not always 100% and you never know what you end up with until you try it, they aren't cheep. The other nice thing about going with a chevy v8 350 (other than the power of course) is that as volvo marine make an adaptor ring for their proppeller drive assemblys for the chevy engines (and they use the same connection in their aquatic equipment as they do in their 700 800 900 series) you won't need to change the gear box. The best gear box that you can have is the 4 speed with electronic 5th gear as this is the strongest box that volvo make for these cars.

Finally, in regards to the ABS system. You will need to change the break drum and whilst you have the engine out to change it you will have plenty of space to change to ABS brakes. Just remember to get the entier breaking system (no need for the break lines but the calipers, drum, and any other equipment directly connected to the drum) otherwise you might find yourself with semi functioning breaks.

What you are asking here is possible but won't be cheep and will take time. You might even be better off selling your current model and getting the standard 6cy one. It all depends on how capable you are and if you are capable enough how much of the work you are willing to do yourself and how much time you have.