Quick ? on crankshaft pulley

im in need of some info my mazda lost power and wouldnt go past 45mph so i checked the timing it was off so i bought the belt and tensioners and come to find out the key that holds the crankshaft pulley was broke and it messed up the slot on the crank so i bought a new gear and key now and im a pretty good welder so i welded the slot back how it supposed to be put the pulley on and i was JUST WONDERING DOES THE PULLEY SUPPOSED TO BE LIKE SUPER TIGHT ON THE CRANK OR IS IT OK FOR IT TO MOVE JUST A BIT????? PLEASE HELP THANKS

Ok....Just an FYI if you weld anything to the crank and it is not balanced and machined properly it can and more than likely WILL be off balance. This can cause all sorts of crazy stuff to happen.
Here is a pick of my block...it was rubbed and is completely junk now...

Then here is the lovely job the previous owner did. Yummy....

As you can see they welded it on there. God knows what else because just while trying to cycle the pistons up and down with a little ratchet and maybe 30lbs of pressure the CRANK broke. Yeah tons of fun. Just an fyi....not saying it will happen but it could. Please keep us posted so others will know what to do if they have the same issue.

yeah damn mine wasnt bad bad i just filled in the wobbled out slot where the key goes and molded it back flush with the crank shaft just to make the key fit properly some idiot JB welded the slot to make the key fit snugg and that shit didnt work so i did everything myself and just got done filling the wobbled out slot with weld so everything is tight and snugg now the guy i took it to lost the lil washer for the outter side of the pulley so now i gotta go get that and some bolts he also lost to hold the timing cover on THANKS FOR THE HELP REALLY APPRECIATE IT pics will be up soon of my truck thanks again