Actually the bolt hole locations are in the exact same spots. The only reason accessory brackets are different for the BBO is deck height. All your accessories will move right over. The other issue is Chamber size, same as what you have and the Edelbrock/Procomp aluminum heads can't run a mechanical fuel pump due to over hang on the SBO. Slight clearance is needed on the BBO for most but the nice RobMc can't be used either. You would need to sell your Performer to Chuck Cole, he was having trouble getting one on here or new and buy a RPM intake. It has meat above the ports and can be opened to the BBO port size. I would also make sure you run 9+ to 1. The J heads are basically #8 SBO heads with a bigger intake valve and port. Similar numbers stock compared to your heads. The Procomps can be bought for $300 each or less bare. I am tempted to go Procomp's on my 403 as well. I have never done it but as has been mentioned Olds did it factory with two different 400 ci motors. But neither were high 7 to 1 motors, lowest was 9 to 1.