pistion slap is caused but wear int eh cylinders. i ahd a 327 that knocked on start up and the high rpms, only way to fix it is a compete tear down and rebuild, i'd get the numbers off the rear of the engine block to verify that you do indeed have a 283 DO THIS BEFORE ordering any parts for the engine, Then its all about dissembling the motor and having the cylinders bored and honed new freeze plugs cam bearings , heads reworked, havignt eh crank checked out at the machince shop so you can get mtching bearings for it. good place to get rebuild kits are
http://www.northernautoparts.com/.
i rebuilt a 305 for my truck i had over 600 dollars in parts (rebuild kit, rods rockers vavles vavle guides, a new crank) ect..) i did all the machine work at school with teacher supervision, but if i had to pay for all that machine work i'd been at close to 1500 dollars, and you can get a gm 350 290hp turn key for 1900, all on what you want to do