I'm running a 15x8 with 4.5" BS (275/60-15) on the back of my 68 Barracuda with no problem (Stock suspension)...all things being equal, you should be fine.
For what it's worth, I'm running 15x6 up front with 3.75" backspace and no issues. If you don't mind me asking, what size front tire are you planning on running. You can check and compare your combination here:
Rim & Tire Size Calculator. Custom Offsets
Here's my combo...you might find as I did that 15x4 front rims are, in my opinion, a bit too skinny and limits your tire choices. I found 15x6's perfect and the 215's fill out the wheel well nicely...but ultimately it's up to you and your taste.
Post your wheel and tire combos.
Your car sounds really good with the Accurate mufflers...but with the Gibsons it sounds absolutely wicked...I have those on my car as well, love the sound and the quality.
You asked for best suspension...if money were no object than I would go with RMS AlterKtion front and RMS street Lynx for the rear.
Reilly MotorSports, Inc.
Since it was a /6 car, I'm guessing the fronts were originally drums and a 5x4 bolt pattern that was at some point converted to a K H dual piston caliper. According to the literature, the 1973 318 cars and above came standard with the new single piston disc brakes and 5x4.5 bolt pattern.
Ride quality is really a personal preference. My suggestion is to start with the front sway bar and stop when you reach a point where you're happy. You will also want to install a rear sway bar to balance the front. if you don't, the rear end will want to step out on you during hard cornering...
I highly doubt it's numbers matching. If it was, then I can guarantee you that the seller would make that very clear. Just like the Green Duster above, It clearly says it's an H-Code car...but nowhere does it say that it has the original engine. If it did, I'm sure he would also make that very...
I laugh at the people who tell people it's going to cost 7.2 million dollars to restore a car when all they have done is write checks to someone and get ripped off by supposed restoration shops. especially when they are simply looking at a picture , have no clue on the actual condition of a car...
Throw out a price and I'm sure he'll counter...eventually you'll find a middle ground that you both may be comfortable with. If you're both worlds apart and he has unreasonable expectations then throw out your final offer and a phone number and walk away...