What an awesome find. I have a 68 Dart Hemi car automatic, its an old drag car so a similar situation. Just from my experience, I would have the entire drivetrain taken apart, motor, rear, transmission. Did it with mine and we found lots of issues that could have been a disaster had I romped on it hard one too many times. Get the mechanicals right first and consider if you want to drive it on the street, track or both.
Some observations, while really cool looking and interesting the roll bar looks sketchy, only an issue if you want to make passes in the car. I personally would get rid of the mag and run a period correct distributor set up for an MSD. Hide the MSD under the dash, will allow for better drive ability and a reliable rev limiter in case you miss a shift. Original Hemi Darts came with a transigniter ignition, cool old ignition but the MSD is just superior in many ways. If it is slick shifted you will be able to tell when you attempt to downshift at low speeds. The catch can is cool and still available from Mr. Gasket. I have one, decided not to run it for now because the lid is lose and sloshing water on the drag strip by accident and getting it on the rear tires is a bad situation. Might get that fixed and install it but just something to consider. I have an aluminum Weiand tunnel ram for my hemi and a magnesium cross ram. I am running the magnesium crossram and I have a replica Hemi Dart Hood that I got from Kramer Automotive. They also made the tunnel ram in magnesium, the magnesium versions are more valuable, check what you have. Dana is a necessity for a 4 speed car btw. If you swap cams consider a modern roller. Since it was a race car you prob have 12.5-1 pistons/pop ups. I run those, swapped in some good billet rods for reliability. Heads, guessing you have iron heads, I drive mine on on the street as well. I have aluminum Mopar heads less weight and less likely to detonate. Painted them engine color, nobody really notices they aren’t vintage. The seats in your car have the welting, 68 darts had no welting. The seats could be original 64-65 Hemi Super Stock seats which had the welting. Check the suspension, see if you have a snubber on the rear end. I would recommend adding one if not. Springs should be Super Stock versions, lots to check. By 72 when it was raced original hemi darts were being sold and parted out some of those parts could be on your car. Just guessing but your FIL may have run it as an A/MP car. Without going through everything associated with these cars I would recommend the Super Stock Hemi restoration guide so you can ID what parts you have, some of them could be quite valuable.
Decide what’s right for you but I would make the car safe and reliable so you can run it hard like it should be and not have to worry about it turning into an pile of parts. BTW either intake Crossram or tunnel will foul your plugs even with an MSD if you just drive it around like a normal car. The fuel will puddle in the intakes until you get some air flow going both are race intakes. There is a Super Stock forum on here if you want more info. As it sits easy 40-50K value, fixed up rebuilt much more IMO. Nothing like the sound of a well tuned and running race hemi. It’s so rowdy you will be amazed. Awesome unique car, congrats