1974 valiant stalls in gear 225 automatic

thanks everybody all good ideas . i forgot to mention , but right before leaving to come back from carlilse . i replaced the muffler with a cheap one i found at carlilse , it was the right size , but no other info . it is new though . is it possible that the new muffler could somehow be causing this ? like wrong backpressure or something , i think i installed it in the right direction for flow . just a thought . only thing changed before the problem started .