Re: 516 Heads
Hi. You might be better off looking for a set of newer heads from a mid-70's 400 or 440 (These will be the 456 casting #). The 516 heads, unless they have been redone, don't have hardened seats in them for unleaded gas use. The have closed chambers as well which is going to give you a 10 to 1 Compression ration, if not higher. This could be a problem with todays gas as well, especially if you are going to be doing towing and stuff. The 456 heads are open chamber and will give you about a 9:1 compression ratio. The 516 and 456 numbers are the last three digits of the head casting number. You can easily find it after taking off the valve cover when you start your search.
The engine otherwise is a great piece. BB Mopars are bullit proof. This engine will also have a forged crank in it. You will get torque nubers in the range of 450 lbs-ft. Don't go spend lots of money on cams and intakes either. If you want even more torque, go get a stock 440 HP cam from your local auto parts store. Look on ebay for a late 60's - early 70's 440 HP intake or just get an Edelbrock Performer. You will be totally blown away when you step on the gas the first time.
If you have any more questions, please ask.