YOU talked about a TPI motor. The BLOCK is ok but the Injection system can have many issues. I was an auto tech and my first line I worked on was TPI GM cars, so I know the TPI system well. It was good for the early 80's but got long in the tooth quick. And yes in a Fiero it would be even worse. The motors ran warm enough in F-Bodies, let alone a Fiero.
Teh Eco-Tech makes as much or more power than the V6 that came in the fiero but gets good gas milage, which the OP seems to want. Let alone much more reliable then a TPI motor and less weight. As such the Ecotech motor would work much better for the OP and what he wants.
It has many issues? Such as? I've had several 350 TPI setups and have never had an issue. I know the pre-91 models with MAF some had MAFs that would crap out, but if you go with a 91-92 it's speed density so no MAF. My current T/A is a 350 TPI with exhaust, custom cold air intake, ported plenum and runners (still need to do the base plate), ECU tuning, under drive pullies is right around 300HP, 450 on the bottle, and that's with stock heads and cooling system. I want to put some ported aluminum L98 heads on it, an LT1 cam and 1.6RR. After that comes the turbo charging.
there's no doubt the ecotech makes more power than the stock motors that came in the Fiero, but personally, having driven a 350 powered Fiero, it's a hell of a lot of fun, and in the fiero, low to mid 20 mpg isn't unreachable with a 5.7L motor.
But, I will say if you're buying a project/toy/fun car and are worried about fuel economy you are going to be disappointed in the end when you find out how much money you wasted getting to the point vs, buying something that already had good MPG.
Doing projects like this are money pits. You'll never recoup your costs.

Would a V8 even fit in a Fiero?
Check my link above. They have FAQ's that list every part and bolt you will need to do a swap.