Need help deciding on a 360

With a 70 Dart, this is easy.
All you need is a stock 360 and a 5spd, and 4.30s to 4.56s

What no 5-spd? Oh that's still not a big deal.

At say 3400pounds, you need about 310hp, the right gears, and the tires gotta hook to get you a decent 60ft. That's all there is to it.
But wait, oh no not again, he wants 15mpg

Ima thinking it's gonna have to be one or the other.
Lets say the 15mpg is more important to you. With a 360 this is easy to do, just not with 4.30s to 4.56s;UNH-unh,nooo
So I tell you what,I can tell you are probably on a shoestring budget. So Just drop any old 360 (one that's in good shape of course) in there with 3.23s to 3.55s and see what happens. Just do not swap out the cam! The only changes allowed are headers, a free-flowing exhaust,a 4bbl,any 4bbl, and fresh cold air. . This will get you 13s and near 15mpg. Cheap easy and quick.And a ton-O-fun with a lightweight Dart.
You may have to slip in some 340 valve springs if you go 3.55s on account of you will be trapping a lil high for the 2bbl springs, and she may want to pump up the lifters. I see this as a 96 mph car at the top of second gear going 6400 with an automatic and 5% TC slip. I see this as just barely into the 13s, if it 60fts well. Just barely. At 3400pounds raceweight this might be an honest 250 hp.
If the power falls off too hard up there, you will have to shift early, and lose a bit of ET, maybe enough to kick you back into the 14s.In this case 3.23s would be better for a two gear blast,bringing the Rs down to 5800 or so, but with an auto and a stock stall, it will be a lil soggy out of the gate. 15mpg should be doable with 3.23s, but iffy with the 3.55s.Yeah I might be lil optimistic here.
But the buy-in here is cheap.
A good running 360, a proper flexplate, a 904, headers, intake/carb,springs,properLINKAGE, gears, LSD, and exhaust. Some of this you may already have.Leave the spare and jack in the pits!
Careful shopping could get you in under $3000.

'Course for 3Gs you might be able to get a turbo onto your teener, make 300/350hp, go mid 12s and make 20+mpgs.......... with 2.94s even. 8PSI! Yeah that's me adreamin'