you can get the 88 but as soon as the mod bug bites you you will convert it to mass air and put a lot of mods into the car when your done

 

My 87 GT 5 speed (bone stock, including orig spark plug wires, mods except for a K&N at the time) got 26 mpg at a steady 55mph over a 325 mile trip with the 2.73:1, and got 29 with a 3:23 on the same trip in the same kind of weather. I think the engine lugs a little too much with the tall gears at relatively low revs. With the 3.23, I'd guess that the computer had a little easier time keeping the mixture at a steady state and the engine may have been at one of those sweet spots just a couple hundred revs up the scale. Around town, it got 12-20 depending on how much massaging the throttle pedal got regardless of which gear I used.
I just finished a bunch of minor mods (it's first set of plug wires and injectors, fuel pump, new filter, new Flowmasters and O2 sensor) but had to return to the 2.73 because the 3.23 diff is going into my 5.0 MGB in the near future.

 

what would it take to convert it to mass air if i decided to do it and how much would it cost.

 

you would need these items:

A9L computer $100
MAF conversion wiring $40
MAF (this is for a 76mm) $200

 

a maf
a A9l or A9p
a different harness or a wiring diagram and wire up the existing harness for mas

 

how many horsepower does this make over one without mass air

 

you will actually loose maybe 2-3 hp because a maf system is a bit more restrictive. but if you plan on doing mods that increase airflow in/out of the engine than you will need it.

a speed density system estimates how much air should is coming into the engine and adjust air/fuel mixture accordingly.

with a maf system air has to pass through a maf sensor. this sensor contains a wire that voltage is applied to. air rushing through the meter cools this wire and is and air/fuel is adjusted according to how much voltage is needed to keep the wire heated.

there are a couple other sensors too, but those are the basics.