Possible maybe, recommended to try it? probably not!
Mid 12s,20mpg and streetable? Not hardly.
Mid 12s is 10.75lbs/hp, and race suspension, or about 9.25 pounds/hp without.
Your slanty 70 Dart is about 3050/3100 empty. Add driver,gear,and say a turbo kit;you might be around 3300. So you are looking at 3300/9.25=360hp with a street suspension.
From 225 cubes this is 360/225= about 1.6hp/cubic inches.
The only way to get this done is to be supercharged.
If you want to build the slanty to make 20mpg you will need to stay pretty conservative. Let's say .8 hp /cid and that will get you .8 x 225 =180hp. So now we have to get to 360hp from 180. 360/180=2.0 so basically the supercharging will have to double your power.
That's the math of it.
Now you have this; the factory rating on the slanty was IIRC around 140. So all the guts of that engine were designed for 140 hp. 360/140=about 2.6, so you are asking those guts to put up with 2.6 times more stress than stock. Will they handle it,IDK.
Then there is that slanty head. Can you jam enough air through that straw of a head to make 360hp? I think it's been done.
Now you want to make it streetable? Well, that kindof depends on what you are willing to put up with in terms of ride quality and engine response.
To go mid twelves is to go about 105mph. To hit 105 with a turbo'd 225 will take a pretty good balance of rear gear and convertor.Ima thinking to make the 20mpg number, you are gonna have to stay with 3.23s or less. Well 3.23s will get you 105@4200 with 27s and 5% TC slip, in direct gear. So now, not only are you trying to make 360hp from that slanty, but you are trying to do it at sub-4000rpm! So no I don't think that is gonna fly.
So the next go-to is overdrive. This will allow you to spin slower on the hiway for better mpgs, and it will allow a better race gear to et well.
So if you can get a set-up with a .71 or so O/D then you can target 105@ say 5000rpm plus TCslip, so say 5250.The power peak then could be around 4800.This looks like 3.73 to 3.91. Let's go 3.73s. These will get you 65 =about 2140, so perhaps a little high but I think still in 20 mpg territory. At the other end 3.73s will get you 105@ about 4650 plus slip or about 4800. That's a lot better. Enough to go mid twelves? Sure, just keep cranking up the boost 'til you hit the mark, or it blows up.Idk man, thats a lot of boost. To double your 180 hp slanty will require at least 15 psi.But the 180 hp engine, even with O/D will have a hard time to hit 20mpgs
Now if you lived on my side of the border where the gallons are real gallons,lol,sure.
So, back to the beginning;
Doable? maybe. Recommended to try? only if you have wheelbarrows of money .....
