E Body manual box in an A body?

When you say it has '73+ brakes, does it also have '73+ lower control arms? If so, a '72 and down sway bar will not fit- the spacing of the tabs is too wide on the '73+ arms; likewise a '73+ sway bar will not fit on a '72 and down K frame...
Did you rear mount the calipers? Front mounted calipers generally do not clear the '72 and earlier sway bar.
JYH is absolutely correct, you have to be careful in your part selection to get it all to work correctly together.
If your steering box does turn out to be a large sector box, I believe you'll need to use a later pitman (and idler) arm, which would require use of a '73 and up center link. Otherwise it should be a bolt-in.