I have done 6 or 7 swaps in A and F bodys.
First if your trying to do this swap with stock parts to save money , you may have to do this;

remove the lip on the k member and leave just enough to reweld .
you really need a c body base pan (400 ci) to get good clearance

you may have to flatten out lip where firewall and floorpan are joined .
reverse the tranny mount so engine will move forward
depending on what type of mounts you used , some mods may be needed to let the engine move forward.
i used manifolds every time and only had a few clearance issues with the 440s
It should not be a hard installation with a b block .