The 401 will for the most be a drop in provided it has the 6 bolt crank flange that will accept the 305 flywheel. The only other issue is in the placement of the water pump housing. If you choose to use the 401 water pump housing then you'll need to move the radiator up higher so the fan lines up. If you choose the 305 water pump housing then you need to eliminate the 3 piece timing gear and replace it with the 305 double roller and timing chain and sprocket, this also requires eliminating the center idler gear shaft which is bolted on. It needs to be capped off with a 1 1/8 expansion plug(freeze plug) in order to block the idlers oil passage or else the engine will loose oil pressure which feeds the front main bearings. It may sound a lot to you but it's not all that bad I did the same with my 478M conversion and it all worked out just fine