I personally would stay with the 350. The 400 is defiantly tricky and with it being a 2 bolt main I'd put a for sale sign on it and let it go. As far as the 383 I have had one of those too and you have to turn the journals down to far to fit the 350 block plus the block has to be clearenced for your rods also. If you get on it to hard the crank has a tendency to twist I can share sum pics of one I twisted at 2grand rpm's at about 55 mph if you need sum evidence. So my first opinion is the 350, second would be 400.