weldwade,
STOP!!
That is a waste of AR500!!!!! I used to make gongs and silhouettes for target shooters out of that and scrap Hardox drops from mines and fab shops. Gun nuts (like me) loved the dinosaur, buffalo and other "dangerous game" critters I would cut out. the "Anaconda" was especially challenging. People pay extra for the fancy stuff, then you can buy some scrap, less crack prone A36.
But to answer your question, ER 70 S with a 250 preheat should do just fine. Especially for something going through heat cycles. Peen your welds to stress relieve.