Chevy sprint starts in third. is fine if shifted manualy

run the fault codes and look for faulty servo/solenoids and TCM
it could also be a oil level or oil condition problem
I suggest a visit to an accredited auto shop specialist for a proper diagnosis before you waste money
if a new box is suggested get second and third opinion and quotes

Kickdown cable unhooked, attaches next to accelerator petal cable.

shift solenoid for third gear maybe bad up inside the transmission pan

Code 81 is brake switch input circuit.The being stuck in third is a limp mode because of a hard code being set.If you repair or replace the brake switch issue you should be fine on the shifting issue.Good luck and let me know if i can help.

85 Chevy sprints do not have ABS Brakes. So the recommended solution posted by WARLOCK61 is not relevant to the problem.

check your kick down cable attached to the carb, it may be out of adjustment and forcing the trans to stay in "passing gear"

this problem is a common failure associated with your shift solenoinds inside your tranny. of course a shop is not gonna tell you this because by the time you wanna get it repaired it is usually too late and have already cause damage to your clutch pack and internal parts, thats the main reason why a shop wont mess with just replacing your shift solenoid (aka valve bodie)

try to lube the throttle body and see if it moves if you can manualy move the throttle at the engine then check behind the peadle for any obstructions if nothing stopping the peadle then i think you can say its a safe bet that it's the cable

i dont think the two things are related check your speedo cable inner cable could be frayed or kink on outer cable if not disconect cable from speedo end and lube it

You are going to need the transmission rebuilt.