would it work to add some correct 90 deg boots (new or used) from another set?

I buy the Taylor brand that you have to crimp the distributor ends on, they have different colors and you can remove the lettering with some laquer thinner or enamel reducer.

I prefer a set that I can cut and fit myself. Can't do that with Firecore since they only sell pre-made sets. Taylor or MSD for me. There is no magic in plug wires, or any ignition component for that matter, no matter what their best sales gimmick is. I've tried them on the dyno and at the track...as long as your wires are in good condition, they work fine.

Just found the measures I provided to customwires ( firecore ), measured per customwires specs, from center of terminal to center of terminal. Measured the NOS set I got to get the firecore set correctly done

Coil---9"
1-----22 1/4"----90º boot ( cilinder 2 )
2-----25----90º boot ( cilinder 4 )
3-----31 1/4"----90º boot ( cilinder 6 )
4-----40 1/2"----135º boot ( cilinder 8 )
5-----43 3/4" ---- 90º boot ( cilinder 1 )
6-----48" 1/2" ---- 90º boot ( cilinder 3 )
7-----55 3/4"----90º boot ( cilinder 5 )
8-----56 1/2"----135º boot ( cilinder 7 )

This is to use the stock wire brackets ( which need to be trimmed a bit to accept 8 mm wires )

Different year 400's used different lengths due to how they ran and different tabs on valve covers. I see later 400 wires on E-Bay often.
Ron