Toro are not normally a belt problem as they use standard profile belts.
First thing did you check the new belt against the old ?
Why did you replace the belt in the first place.
Belts come from the factory in bundles, the wholesalers when make & fit the identifing sleeve.
And yse sometimes the wrong sleeve gets put on a belt so they always get checked.
Now that is an old mower and if the belt it loose when you engage the blades either you have routed it incorrectly or parts are flogged out.
Pull the tension arm off and I will bet the hole is flogged out oval.
The owners manuals tell you to oil them but no one ever does, I give them a good heavy coating of dry lithium grease on every service.
Now double check all the spindles & pulleys.
A little nick in a pulley will throw a belt as will a dud bearing.
Everything should turn freely and QUIETLY.
The tension spring can go weak over time and also cut grooves in the mounting points allowing it to go loose enough to throw a belt.
Then here are all of the pivot points on the deck mounting which can allow too much mal alignment for the belt to run.
When you engage the blades, the deck will twist & move towards the engine again causing excessive slack in the belt.
Check for wear on the pulleys themselves, the V grooves wear allowing the belt to sit deeper in the groove effectively reducing the length of the run , thus the belt runs loose.
On a lot of old mowers I run slightly shorter belts to compensate for wear as mower parts are dear as poison down here.
Now if you have to mow before you have the time to check the deck , or get replacement parts then check the belt run at different height settings.
Where ever the belt run is the straitest, engage the blades there, then move the deck to your usual cutting height.
Just remember this is a work around, not a fix.