Yes easy build to get those #s. HP& TQ:) I have a 340 stroker 416 that does more than that, but it AINT no 20 mpg engine. Is way to stout.
The thing to understand too, it a long stroke means faster and longer piston travel and speeds, ths more and sooner ring wear too. Also rod bearings get more pressure on them as well.
You will want a 400 or 440 with a stroker, a decent size cam, small overlap and low on duration with fairly decent lift though. I would go with a comp cam roller, most likely on the .550-.590 lift range. This big engine is going to need a lot of air.
You could even setup a larger 800 cfm Thermo quad jet it correctly on a 2 plane intake, run a mopar electronic dist with vac advance, over drive yes and a 3.23 or 2.94 gear is enough with the torque.
2000-2300 medium loose convertor.
Decent set of even iron heads, with at least port matching, 5 angle valve job, 10-10.5 compression with the big cam.
Or run an air gap with an 800 AVS and tune it.
Dont get to crazy on the extra large primaries, stay like 1 3/4 or 1 7/8 into a 4" collector.
Again to copy what was said, on a stroker it is a coffee can size piston moving 4"+ and will suck the hell out of the intake and carb on the intake stroke, same applies for exh side and wasted a/f etc.
So you need to have it geared for no more than say 16-1800 rpms in high aka over drive at 65 mph .... it should work and not bog. Call comp cams and talk to them, they can do a custome profile and give you some tips etc.
I have a late model SRT with a 6 speed, gets 28 on the hwy, has 6 gears, 5th and 6th are over drive and even with 3.92s, with the huge 20" rims it is turning only aboyr 1300 rpms at 75 in 6th and 2000 at 75 in 5th.
When you have enough TQ it will pur and idle down the highway.
But if you even think about putting your foot in it, 11mpg would be good.
PS my Dart with the stroker 416 maybe gets 8 mpg hwy and about 6 in town if it is lucky:)
My car is not a dailey driver and I run 3.91s on a 26"tire with a special low 1st in the 727 so it is like pulling with a 4.56 and I run 4500 stall.