I have done the A body RB block change about 8-9 times . used a 400 base pan with center sump . i also cut off some of the lip on the k member and then mig welded it. I also bent back the lip on fire wall/floor pan to give transmission a bit more clearance. Always went with Hp manifolds as I hate headers. Make sure engine is set as low as possible because you could have some clearance issues with carb /breather / hood. may have to use shorter spacer on fan depending what rad you have. Good Luck .