In all probability you have the 6-3/4" rear end. Unfortunately you would have to remove the ring/pinion and have the new gears properly set up. That will not come cheap.
It would likely be far cheaper just to swap the whole rear end. Here in So. Cal. the Pick Your Part price is $100 Drum to Drum (meaning complete). They have 50% off sales monthly. But before that seems like $50 there is a $15 core, and tax and environmental fees that probably push it to $80. If you get an 8" then you can just swap the center section with easy. I think I paid $45 out the door for my 8" 3.40 center section.
BTW, just changing tire diameter has the same effect as changing the rear gears. The Pinto rear wheel well sure has room for a bigger tire. There are calculator all over the internet that can give engine RPM for a tire and gear size. You can have a 185-70-13" tires with 3.00 gears and it might be equal to a 3.40 with 225-60-15" tires. This is not actual, but I'm throwing it out there for understanding. Again, use the calculators.
Lastly what ratios are you looking at?
The 8" came with 2.40, 2.75, 2.79 3.00, 3.40 and 3.55.
The 6-3/4" was 2.73, 3.18, 3.27, 3.40 and 3.55.
These numbers were from Mustang II.ORG for 1974-1978 cars. It is just my opinion, but the best "average" ratio would be the 3.18/3.27 for the 6-3/4" rear if that works for you. Lastly if you are getting 105 HP out of a 2.0 I'd assume it is in the upper RPM range. For normal driving you may have lost the lower RPM torque to use numerically lower gears. As the saying goes you don't get something for nothing. But there are minor variations to maximize what you have. I ran (for a short period of time) 225-60-16" Turbo Coupe tires in my 2.0 with 3.40 gears. Can you say BOG?