... I'm thinking of using a Spectra 100# line for the bridle. Is that the rght material. And lastly what knot is used to attach the bridle? Thanks. :'(
Nope, pure Spectra is very slippery and may not hold your knots or adjustments well, some beeswax may help though if you choose to go that way.
High performance bridle line is usually Dacron sheathed Spectra, the Dacron provides cushioning for the knots the same as sleeving on your lines and holds knots better, but Gone with the Wind is currently out of stock Gone with the Wind - Bridle Line
due to Laser Pro's dropping of the product.
I would probably just go with a good quality Dacron/Polyester line in the meantime, GWTW has a closeout on 130 lb white for $3
right now, a little stronger then the Desire requires but workable, otherwise the Black Dacron in 80 or 100 lb
should be a suitable replacement.
Many bridles just have loops tied into the ends with a simple half hitch and are then larksheaded over the spars during assembly. There is a knot Ken of Blue Moon uses that is tied after the kite is assembled, I don't know the name though to find the instructions, hopefully somebody else who did his workshops will pipe in with it.