Question: Assume there was a super advanced alien race that for some reason, wanted to add a new planet, of lets say 1 Earth mass, into the Solar System. Where in the inner Solar System *(The part of the solar system inside Jupiter's orbit, so asteroid belt is included)* could we put this planet, so that it would have a stable orbit for billions of years?

*Note: I would preferably not want have this planet orbit closer to the sun than Mercury, and would like for it to affect the inner planets as little as possible, although I care more about the first condition rather than the second.*

*And also, the planet needn't be habitable*

Answer: 
There is a book about the possibility of human habitable exo planets around other stars. *Habitable Planets for Man*, Stephen H. Dole, 1964.

<https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf>

On pages 46 to 52 Dole writes about the spacing of the planets in the Solar System as it was understood in the early 1960s. Dole estimates that the forbidden zones around the planets take up ab about 50 percent of the volume of the Solar System, leaving about another 50 percent which might possibly be occupied by other planets under some conditions.

According to Dole's calculations, the more massive the planet, the larger its forbidden zones, and the farther the planet is from the Sun, the larger its forbidden zone will be. And the more eccentric the planet's orbit is, the larger its forbidden zone will be. The mass of the star is also a factor.

Table 8 on page 50 gives the inner and outer edges of the forbidden zones of the planets. As nearly as I can read the tiny numbers in the table, the gaps between the forbidden zones are wider than the forbidden zones for the inner planets.

Figure 18 on page 51 indicates that there is room for a roughly Earth mass planet and its forbidden zone between the orbits of Mercury and Venus, between the orbits of Venus and Earth, between Earth and Mars, and beyond Mars. It might be possible to put two Earth mass planets and their forbidden zones between Mars and Jupiter.

None of the forbidden zones overlap, except for those of Neptune and Pluto, and the orbit of Pluto is tilted considerably compared to that of Neptune.

I note that it would be unlikely for major planets to form with orbits where their forbidden zones overlap, because those overlapping forbidden zones would interfere with he formation of the planets out of smaller objects.

But if aliens build a new planet in our solar system by using spaceships to move smaller objects into place and assemble a planet out of them, or if the super advanced aliens open a portal to move an already formed planet from a distant star system to an orbit among the already formed planets in our solar system having partially overlapping forbidden zones might not matter much.
The planet would be able to be the most stable in a orbit between earth and Mars because that the biggest gap between notable celestial bodies of the inner solar system. So gravitation pull of any bodies would be most minimal hence being able to keep a steady orbit as long as the sun lives.

Edit:I apologize by I messed up(dyslexic). It rather crowded between mars and the belt you best bet would be between earth and mars.

Another solution would to have an orbit perpendicular of all the other orbit since most bodies in this system tend to stay on one plane.