What do you think the best power/drivability combo is?

Unless you spend a lot of time and money on the rear suspension and tires, even 375/400 blows the tires off, in second gear, at 50 mph, with 3.55s. That's an LA 367/ 230* cam, Eddies and 180psi. And same engine goes 93 in the 1/8th with a 2.44plus 60ft..........@3467 pounds.
so I gotta wonder about the 600hp number.
Oh and same engine with, with a 223 cam,now 190plus psi. I got as much as 32 mpgUS .
You asked "best power and driveability" and later asked about fuel economy. No budget talked about.
I am only real familiar with small blocks, so
I'm gonna vote 360 with a small Solid flat tappet cam of 225 to [email protected], and 190 to 200psi cylinder pressure,lol, and a close ratio 5-speed manual, with an overdrive.
If you wanna keep that A500,that's a bit of a compromise . As you already know, the easiest way to mpgs is a low cruise rpm.Which is easy to get with that unit. But the splits are .56 and .65, so it wants a low-rpm/ wide LSA- type cam, or an engine with a lot of torque.
If I were faced with this restriction, I would use the 5.9 with a 3.79 stroke, aluminum heads,a small solid lifter tight-LSA cam, and cylinder pressure at near 190psi.
That makes 387 cubes, a 4800/5000rpm peak power, mega torque for the A500/ stock TC; and I'd run 3.91s, for a cruise rpm of 2100 with 28s..I'd run a dedicated spreadbore, metering-rod carb on the hiway, and MechanicalSecondary DP sport carb the rest of the time. It only takes a few minutes to swap them. Or a really big spreadbore carb; like a TQ or "goggle-valve". If you got the bucks for an EFI, that solves this nicely.
The toughest decision to make will be the overlap period. Of the three cams I have run, all with similarly high cylinder pressures; My best mileage was made with cam that had 53*. The 292/508 was a disaster in the fuel-economy arena at 76*. My current cam at 61* does not do very good, despite 65=2230 rpm gearing, and at up to 60* cruise timing. So I'd have to say the sweetspot is closer to the 53*, or less.. The 32mpgs was done with 65=1590, gearing.
Those 3.91s will put you at 65~5100, so you won't need much cam, the 225 to 228 is plenty to hit this mark. On a FTH you can get that perhapa as small as, say a 262/268/108 with overlap of 49*
With the above reference points;
Mr Wallace says; With an ICA of 58*
Static compression ratio of 10.9:1.
Effective stroke is 3.11 inches.
Your dynamic compression ratio is 9.12:1 .
Your dynamic cranking pressure is 189.51 PSI.
at 800ft elevation
V/P (Volume to Pressure Index) is 180
Let that simmer for a bit.

Hyup you're gonna need a way to keep the tires from self destructing; You're gonna be saying " boy, they don't make tires like they used too....."

That 262 cam could be had as a 222 *.050 as a FTH, or as a 216* as a HFT

I don't know
anything about big inch engines, but I know a VP of 180 is almost 20 numbers higher than my 223 cam was......and it was a tire fryer.
So you could easily give away some of that torque, either in cubes, or to a bigger cam, although, I can't see either until the traction issue is resolved.
Oh yeah with that 387, you don't really need the 3.91s; you could run anything you want back there. I chose those 3.91s to optimize your 60mph trapspeed at about where the hp peak is and for the 2100cruize-rpm.That is the compromise with the wide ratio auto-trans.
I chose the 108LSA because as a streeter you are a 2-gear run to the speed limit. Your engine will not feel the loss of power on the 1-2 shift because it has more than enough power to keep right on spinning in second. Yeah it might get a little sluggish on the 2-3 shift, comparatively speaking, but that shift does not need to occur until 81 mph/6100rpm. And the drop here is to ~3960, so as a streeter you would be fine. And finally, with just 49* of overlap, the headers need something to work with, so I gave 'em a 108.
The same 262 cam on a 114LSA would lose 12* overlap, drop 10psi cylinder pressure,and 17 points of VP, and lose efficiency in the extraction mode, and the rpm of peak power would drop in the neighborhood of 100 to 150 rpm, taking something like 10 or 12horsepower with it.
High Compression Small block is all I know.