Need advice help.

Backing plates? If you mean splash shields then those are optional. Modern cars do not use splash shields. If you leave them off it is less unsprung weight. They may actually hold-in heat more than they dissipate, I don't know.

You did not mention caliper mounting brackets. You need those plus the mounting bolts.

I have a 73-76 spool mount K in my my 68 and this is what I did:

Your present steering box will work fine. For some reason the 3 mounting bolts like to loosen up so be sure they are torqued correctly.

I used the 68 center link, pitman arm, and idler arm with the newer K. You cannot mix 73-76 parts with 67-72 because the taper on the center link is different. One style has the idler and pitman mountimg from the underside of the center link and the other style has the idler/pitman entering from the top. I don't know for sure if using 73-76 lower ball joint/steering arms with the 67-72 center link junk affect the geometry or not. I was pressed for time right before the Nats and did not have a 73-76 centerlink, idler or pitman on hand.

I used 73-76 strut rods with new Moog bushings.

I used the Moog problem solver offset UCA bushings installed such that it gives more caster. Do NOT align the car to the old alignment specs and use the same setting for both sides. I used 2* positive caster, 1/2* negative camber, and about 1/16-" toe-in and the car tracks hands off on the interstates.

Make sure you install new LCA bushings while you have them out. I used Moog rubber. The only poly I used was the sway bar mounting bushings and end links.

I used the 73-76 sway bar because the mounting is different between the two series. The 73-76 passes through the K. Also the sway bar end link tabs on the LCA's are in different locations.

Buy a MC for a 73-76 Power Disc brake car but run it manually using a manual brake pushrod. Make sure the pushrod is retained in the MC piston. This gives you a 7/8" diameter bore MC and the feel is excellent. No need to run power brakes unless you have some physical limitation. (saves weight and less chance of problems) I have a 110 pound female friend and she has no problem with the manual 73-76 discs and the 16:1 manual steering.

I re-plumbed my brake lines and omitted the safety/distribution block. I used a Wilwood (Summit) adjustable proportioning valve and while I was at it, installed a line-loc. The safety switch turns a warning light on the dash when there is a leak but you'll feel it anyway because the pedal will go down. I had a minor leak after I first made the swap and the reason I knew was because I did not have a 100% rock hard pedal when holding it down.

I collected all the parts before starting the swap. Your results will vary.