Simple. The mod part you are showing is the SCREEN Stability part. The Screen lead going to pin 1 is predicated on the new resistor being placed on pin 1 to pin 4, instead of hanging it off the jumper. Totally separate from the IBBA Mod. You are correct in that the 1/2K 7W resistor is R143. This is separate from the IBBA.
For the IBBA you have 6 Wires
1.) POS Wire to the BIAS CAN CAP (NEGATIVE LEAD!) Remove C86,R131, & R132 from the Circuit as the IBBA incorporates replacements.
2.) NEG. Closest convenient Ground.
3 thru 6 are the leads to the OUTPUTs at the Grid return resistors (330K or if you did tone lihe mod already 200K-220K). One lead per tube. Separate the 330K resistors on the old "T" strip (replace the "T" Strip with a 3 terminal strip, and each grid return resistor to a separate terminal on the new strip).