3 step rods were used with the AVS with tall jets and domed rod covers and the AFB used a 2 step rod short jet and a flat cover. Just be sure use the right groups. If you can afford it buy the Edelbrock Strip Kit and use the flat top covers.
Yes high jets and 3-step rods are listed in factory part and repair manuals for AFB back as far as 66 (haven't looked back earlier).
Are the AFB and AVS rods the same? I think I remember the AVS were longer but don't remember for sure.
I'll get back up in the next week or so D if you have no luck. I think he said he had a few 273 AFB's there.