Ok, this is to the best of my recollection....and I am not in anyway taking credit for this mod as I was merely following instruction from Tony Vincent and someone else from the thread you referred to.
Mark the BT70 with two opposing vertical lines at 180 degrees. These are for lining up the motor hooks later.
The BT70 section butts up against the star ring(#2). Go ahead and glue in the stuffer tube coupler as per instructions
While holding bottom of hook flush with bottom of BT70, mark the place on the vertical line where the top of the hook ends. Now, cut the little slits for the top of the hooks slide into(typical motor mount stuff). Hold hook in place with tape. May want to reinforce this area with CA. Repeat at 180 degree mark.
All tank tubes need to be marked and cut to fit around the BT70. One by one, hold each tube in one of the star ring notches and make a little mark on each side where it touches the BT70. Use a straight edge to extend those marks lengthwise on the tubes. This will be the width of your tank cutout. I don't recall what the length for the cutout ended up being. You make have to refer to the instructions to see where the tanks are supposed to fall and make your mark accordingly.
The inside of the aft centering(#4) ring needs to be cut to fit around the BT70. Do this while the cloverleaf looking punch-out is still in the middle. I used the cloverleaf as a reference and drew cross hairs to find the center. Then used the circle cutter set to the BT70 diameter to make the cutout.
Motor mounts:
You'll need to modify some 4 X 18mm X BT70 and CR5070 rings (available from Fliskits, BMS) or fabricate your own rings to fit inside a BT70 coupler. You'll need a set of each. Matte board works well for making rings if you decide to make your own. I cannot stress how handy the circle cutter is for this mod and just a great tool to have around. If you already have one then you know what I mean.
Follow regular motor mount procedures. Mount assemblies get glued into each BT70 coupler. Make sure top rings are recessed by about 3/8-1/2 inch and bottom rings recessed far enough for the motor hooks to operate. Glue TR115's to the top of each mount.
Mark the top of each BT70 coupler at 180 degrees and cut 1/8" X 1/4" notches at each mark. The notches may need to be slightly deeper...you'll know when you try to slide the mount in. The notches allow the coupler to slide past the top of the motor hook when it gets inserted into the BT70.
I hope this is clear enough. It starts making more sense once you actually start into it. I was pretty nervous about making this mod but knew doing so would be easier and more cost effective than building two birds.
It is also important to note...Carl @ Semroc noted marginal stability with this model and suggested being mindful of adding extra weight in the rear. You may want to few grams to the capsule just to be on the safe side. Having said that, I've seen Tony's fly without any extra nose weight and it was picture perfect.
