I think you will need a rear sump pan (like your 300), but I am not an expert on the big cars. Most Buick 455s had a rear sump pan and this will fit the 400/430. You will also need big engine frame mounts (fabricate or adapt from an Electra/Wildcat with a 430 - 67 I think), big block motor mounts, big block springs from a similar year Electra/Wildcat, a ST 400 transmission, driveshaft changes, a new (bigger) fuel system and a different exhaust system. Then you still need the bigger radiator and all the accessory drives and mounts from the 430 (PS, AC, water pump, pulleys, belts). I can think of those obstacles off the top of my head, but there are more that will need resolution and/or purchase. It would be cool in the end, but may be more of a project then you think.