I am going to convert my 12 bolt 331 open rear end to 411 posi. i already have the gears and am looking for the most economical and practical way to covert to posi. i do not use the car for daily driving and never take the car on the interstate. all i use the car for is fun. i may even take it out to the local dragstrip and bracket race some. i know spools are cheaper but i need opinions. i can get a rebuilt GM posi unit for $275.