Re: big block swap
The Big Block is called a big block for a reason, it is mucho bigger than the small block (in all three dimensions, plus weight). Recognizing this Chevy redesigned the heater core box on the F-body (and the X-body) originally built with the SBC in mind; to clear the increased dimensions. On the Y body (we call 'em Corvettes) they had to also had to "dimple" the driver side valve cover to clear the brake booster in order to squeeze it in.
These parts are readily available from restorers (one of whom graciously helps sponsor this board). I seriously recommend you purchase the correct parts (such as BBC frame pedestal engine mounts, the TH 400 cross member to replace a TH350, if swapping in a bigger trans etc.)
Not only will it save you hours of grief (SBC mounts put a BBC to low and push it too far back) dealing with header fitment, and distributor clearance: but any one who knows what a BBC installation should look like will see your engine is in the wrong place, you will also have drive line vibrations, and a host of other gremlins that crop up.
I ran Morroso Tall red aluminum turkey trays for years. They were so light and flimsy it was easy, even with stud girdles, to get them on and off. Of course they leaked like seeves, but clearance wasn't an issue.
Larger Dave (the Voice of Engine Swaps Past)