How would the HD Guide appear on an old 4:3 HDTV

You seem to be mixing a few things here.
You set the TV ratio to 4:3 or 16:9.
These either give you letterbox or pillarbox.
The HD guide currently outputs in 480 resolution [too], so these questions shouldn't be.

Beerstalker said:

Most of the old 4:3 HDTVs (most of them were actually HD monitors, they didn't have ATSC tuners) will automatically switch to a letterbox format whenever they get an HD signal. My Hitachi would use grey letterbox bars top and bottom.

Click to expand...

"HD Ready" TVs can be a bit strange as to how the ratio and format settings in the receivers work.
I ran across one recently that was a 4:3 and when using composite inputs worked like any 4:3 TV, but when changing over to its component inputs "everything" was backwards, where the receiver needed to be set for a 16:9 ration and the format only was correct with "original" as the setting. All other combinations caused the image to have some type of distortion.
"This TV" needed to process all of this and not have the receiver do any.

"Of Course" YMMV