New to me Suburban, and I already screwed it up.

I just bought a Suburban, the yellow, mid 70s model from what I can tell.

I got the mower deck hooked up, then when I went to engage the blades; it seemed to put a LOT of tension on the pulley on the side of the engine. I immediately disengaged and disconnected the belt. Now on startup, or when I release the clutch to drive, there is squeal/bad noise that comes from the motor, or possibly that pulley. It also seems to have lost power when in drive, but the motor itself seems to be running correctly, its just that noise when I go to drive.

Any idea on what I have done to this thing? Or at least some troubleshooting steps?

HELLO
FOR STARTERS WE DON'T KNOW EXACTLY WHAT MOTOR YOU HAVE. #1 QUESTION ARE YOU SURE THE ENGINE IS FULL OF GOOD OIL. IS THE DECK SPINDLES FREE AND SMOOTH, IS THE 2 MULE DRIVE PULLEY BEARINGS GOOD? THE MULE DRIVE ENGAGEMENT ROD SHOULD BE ADJUSTED TO JUST SNUG UP DECK DRIVE BELT WHEN ENGAGED, SHOULD NOT HAVE A LOT OF PRESSURE ON THE ENGINE PULLY. YOU ARE TALKING ABOUT PUTING IN DRIVE PLEASE EXPLAIN. NEXT CHECK THE CLUTCH DELT TENSIONER V PULLEY BEARING VERY NEAR BACK OF ENGINE, ALSO THERE IS ADJUSTABLE FLAT BELT PULLEY FARTHER DOWN NEAR TRANSMISSION THAT MAY BE FROZEN UP, CHECK BEARINGS FOR SMOOTHNESS. IF YOU CAN START UP ENGINE WITH CLUTCH DOWN AND RUN OK WITH NO NOISE, THEN RELEASE THE CLUTCH AND GET NOISE IT SOUNDS LIKE YOUR ENGINE IS OK, ALSO HOPE TRANSMISSION IS FULL OF GOOD 30W OIL OF YOUR CHOICE, 5 QTS. AND A BOTTLE OF STP ENGINE TREATMENT. SOUNDS AS IF YOU ARE NEW TO SUBURBANS, BE PATIENT AND ASK QUESTION, GREASE AND SERVICE THAT MACHINE AND IT WILL MAKE YOU HAPPY.
GOOD LUCK AND WELCOME
JIMMY

The model number is 917.25743. The internet tells me that it is a 1974 ST/16.

To clarify, When I bought it it was running fine. After trying to but the deck belt on it, I believe too much tension was put on the pulley that comes off of the engine (The main drive pulley.) The engine seems to be running fine. When I let off of the clutch and the drive belt is tightened, it now squeals some and seems like it isn't transferring the same amount of power that it once was.

The oil was recently changed in the engine, not so sure about the transmission, but in regards to my problem I think it is isolated to the pulley/shaft on the engine. (albeit, I do need to go through my mower deck before I hook it up again and make sure all the pulleys and spindles are free.)

The shear pin (or key) got destroyed in my haste to get the deck hooked up. That is all back together and running mostly fine. My problem now is that the cotter pin that holds the clutch spring onto the idler bracket is bending back, so that when I let out on the clutch and put tension on the belt the idler pulley is bouncing up and down. I don't know if was like this when I bought it or if I messed it up from putting to much tension on it. I believe that one I get that tightened up all will be well.

Then it will be on to going through the deck.