Orielly's Zerostart, check different numbers 3100043 oem 1000 watt, 3100045 smaller 600watt, 3100042 smaller yet 400 watt. All these confirmed fit!!!! Price goes down to $25.00ish on one.
I use the 400 watt one but the 600 watt is cheaper.
If you have a pre 85 block and INSIST on using the heater try the 3100042 400 watt