The Stage IV Millenium II Hemi heads dictate the need for the size of the notch. They are about 3/4 higher and a 1/2 inch wider than stock heads. The valve covers are taller and wider than stock too which adds to the problem. The Mopar Performance aftermarket aluminum block is a little wider than stock too so the stock steering box is jammed tight to the left frame rail on my '68. The wife's '69 is getting an alterkation unit and custom steering column after paint so that won't be an issue.
The trans tunnel area needs a lot of massaging due to engine set back. My '68 has a hand fabbed /removable trans tunnel which is taller and wider. The wife's '69 "street car" hasn't got that far yet but it will be hand fabbed (larger) and removable as well.
I now have a stock Hemi plastic mock up block (with valve covers) I use which helps out a lot as far as locating the motor. If I can stick my hands down between the shock tower and valve cover I know I'm good.
