Let's start by clearing up a couple of things related to terminology.
Case garden tractors have a separate choke lever and throttle lever that are located on the right side of the dashboard. On the left side of the steering wheel, there is a TRAVEL LEVER that controls ground speed as well as direction of the tractor.
That Lever should move very smoothly throughout its entire range of travel. If you are encountering hard spots, then something is wrong. Sometimes it helps if you push downward on the lever first, before attempting to move it in either direction but it shouldn't be necessary.
I am assuming that you replaced the Neutral Safety Switch (NSS) which serves two purposes. When the tractor is wired properly, the NSS prevents the engine from being rotated by the starter UNLESS the Travel Lever is absolutely in the Neutral position. The serrations on the nylon block are there to hold the Travel Lever wherever you want it to be so that you do not have to keep your hand on it to maintain ground speed.
The small rod that presses against the NSS also wears. It should be perfectly round but it will become flat over time and perhaps this is the problem you have. That rod is digging into the NSS block instead of sliding smoothly over it. In future, please provide the serial number of your tractor when asking questions so that it is possible to see which "build" you have. The 448's were produced from 1980 until 1988 and changes were made during that 8 year period.