Standard battery isolator. I suspect your getting wrong parts because your mis-identifying the part. It is there to allow the alternator to charge the house battery and block it from discharging the chassis battery when parked.
Start looking online or call the good folks at Texas Industrial Electric. Give them all the info from the old part and they should get you a better quality replacement.
First you will need to get smog checked. (27 dollars)
Then you will register and license it as a house car (motor Home). Fee is based on the price of the vehicle at time of license.
You could possibly get away with putting a passenger car plate on it, but is it worth it?
IMHO you would be wiser to yank the 350 and install a 4 bolt 350 truck motor built up. If you already have the 4 bolt truck motor, pull it havd have it built. 3 to 4K to get it built vs 8 to 10K to find suitable motor and trans and then trying to shoe horn it into the chassis seems a no brainer to me.