Chevy 350 small block has been inserted in quite a few volvo's. I couldn't tell you how easy it is, but it didn't look TOO bad....
Get a small block chevy and a 350 turbo tranny, THE two cheapest and most easiest to find engine and tranny combo in the world. Its a no-brainer in my opinion if you want to go cheap....and can make a lot of beans with very little money.