A bike has a 7 speed indexed shifting set up for the rear. The rear wheel broke and I picked up a cheap 8 speed wheel in the used market. When I tried to set up the the drivetrain, I had hoped to use the all the easier gears and leave out the hardest one because I am too casual a rider and just chill on the way down instead of pedaling.

Can't get it to work though, I can't screw in the derailleur deep enough so that it would stop at the second hardest gear, and the shifter just can't hit the biggest gear consistently. This is by design right? But I didn't seem to have this problem when I did a similar thing on an old 6 speed bike bike with a 7 speed wheel, I used the friction shifter to do 1~6 on a 7 speed cassette.

If this is indeed by design for the indexed shifting, I wonder if I can go get a spacer so I can do a 7 speed like 1~6, 8, and skip the 7th gear.