jon you cant go wrong going bigger than you need. my chevy has the big 6.3l gas engine before the upgrades for more towing and pushing power i was getting 17-19 highway. for plates go get special farm plates 20 bucks a year but the down side to that is they dont quailfy for the passport parking.
real easy if its a older model 350 chevy it will have 4 valve cover bolts 2 on the top 2 on the bottom if its a vortec it'll hace 4 through the center of the cover