Depending on which caddy chasis it is will determine how much of it you can use. The posi may be for an 8.5" differential and the Vega may be either a 6.5" or a 7.5". Same issue may be with the spindles. Is the caddy a B, A or G body chasis?I understand the cadi's a querky setup but I had rebuilt the car as a teen w/ my dad and then it sat for years and the body rotted around all the brand new drive train so I have all of the suspension w/ posi just sitting there. I'm a young family guy so tight purse. Strings. But the axle fits right in by swapping mounts and the caliper brackets are an easy fab I've done on a previouse project.
The heads for the 3800 Seires II engine will bolt on but from there you will have to make everything from scratch. The original Buick V6 RWD blocks are off center design and the Series II blocks are all on center. What that means is that the bores have been shifted to center the bores better on the crank journals to reduce side load on the rods.As far as the motor I'm not sure how to make it breath better. I'm not fond of transplanting f/i harnesses so I'd prefer to keep it on a carb setup but to keep cost lower I know it will have to be force inducted. Someone had told me though its a fuel injection swap that the top end from a late 3800 (1999) supercharged would fit the block but I'm skeptical on that.