To Vacuum Advance or Not to Vacuum Advance

there is an easy way to prove yes or no
Get yourself a dash-mounted, dial-back, timing box.. and start experimenting.
Stick cars respond well to Vcans because with big cams and plenty of compression, it is easy to run too much advance at low mph, and then the engine get's jumpy, because of the very powerful power-pulses. Of course that jumpiness gets into the chassis, and then into your right foot and right-quick the car is doing donkey-kicks.
But a 3500 Tc will suck all that up, so you can run a lotta mechanical advance with that. And that leads to the Vcan being less important... on the street, maybe.
A good convertor will still pull pretty good below it's rated stall, and some combos will like a lotta timing... that you cannot get with the mechanical only. The way you can tell the engine likes it is because it pulls harder with less throttle, and the engine is smoother.
The thing about the engine is this; the pressure spike from the expanding gasses needs to be delivered to the crank at about 15* ATDC, for optimum transfer and fuel useage. And every rpm and load setting requires a different ignition start-point to make that happen; and it is impossible to achieve on mechanical only.Impossible.
The closer to optimum, the less fuel will be required; because firstly the engine is not fighting itself and secondly less fuel is going out the tailpipe either unburned, or with still a lot of energy in it.
I know with that 285 cam, your powerstroke is already very short probably less than 100*, so trying to zero-in on this will be tough, BUT it still behooves you to try, cuz the engine will be much happier. and using less fuel.
In your case, if running iron heads, your pressure is gonna be almost extreme for pumpgas so IMO, I doubt you could use the Vcan very effectively with that 3500TC.
But with aluminum heads you're waaay under pressured so Have at it; get yourself the biggest Vcan you can find or just mod yours up to 20* or so. It will not be too much on the spark-port

But in either case, you may have to, or dare I say; be able to, take some mechanical timing out, if she seems a lil jumpy on what she has now.
The timing curve in the D is there strictly to satisfy WOT timing; it will NEVER be right at any other load setting. Making it right, or getting as close to it as possible, is the job of the Vcan. You the tuner get to marry the curves to satisfy as many points as possible
Happy HotRodding