You must be racing some slooooooow mustangs
But I don't think you want to go to a motor plate, harsh ride and not really intended for a street car. The best way would be to fab the mounts, you are going to need to come up with some kinda of supports for the motor since it is so much narrower, make sure the drive line is on the properangle like a stock car would be.
I think some of the 80-86's came with a v6 not sure what years or what displacement but you may want to look into getting one of those and maybe the K-Member and mounts if it will work.
I think it would be much easier to drop a 5.0L in it and make a lot more power, I know you want to be different but it will be a lot cheaper as well.