I've done it twice . If it was me...
I would use a modern 12 volt ,Bosch type relay ,!2 $ on ebay. if you do not like the looks, put it behind the dash.
12 volt solenoids show up on ebay all the time from $50-$300. Wait till what you want shows up.
Change out the 1 gauge battery wire for a 4 Ga. It will drop some what the voltage to the starter.
I also changed a HJ over to a 6 volt ,60 amp alternator with an Optima Battery. It performed as good as any 12 volt change over and was a lot less work.