You would have to change out your harness from the helm to the motor, use the complete engine and drive harness from the volvo motor , a Merc 496HO or get a complete MEFI set up from Flagship or Arizona Speed & Marine. The exhaust will interchange straight across, but if you have the old style log manifolds I would opt for a new exhaust system, many BBC types to choose from in all price ranges. Should not be a difficult change out for a motivated boater/gearhead. Make sure the drives are up to the new horsepower and torque levels of the 496HO level. The stock 496HO makes 420-425HP on the dyno and the torque comes in about 480 ft./lbs at 3900 rpm.
Hope this info helps.
Ray @ Raylar

I was told by some my stock boats harness will plug into the new motors harness. The motors are completly wired. Is this true?

I had a 93 454 and mine plugged right up. It is a great swap.

Did you go with the red engine? How was the drivabilty after the swap?

Yes I went with the red engine. If you look in the classified section there are some more of these engines available at great prices. You use your old coupler, bell housing and engine mounts be sure to use metric bolts on the coupler. I picked up 14 mph.