Not hard to change. Remove the right footrest and belt guard. There is a service manual that covers the 1978-79 B, C and D-Series that applies to your tractor also for the most part. You can get it herehttps://lookup3.toro.com/request/request.cfm
Use 91-12K802 for a model number and type the word none in the serial # box.
The belt replacement is on page 5-2 and the pto brake adjustment is on page 9-1. You can use a business card in place of the .012 feeler gauge.
Your 1985 312-8 is very similar to the C-120, C-121, C-125, C-160, C-161, and C-165 8-speeds.