By way of prefatory remarks I will include this Ibn Ezra on the verse mentioned as a precursor to any information below, and by doing so we can then remain level-headed.
ומי יוכל לשער כמה זהב צפוי כל כלי המשכן
We cannot really properly assess the gold in the Mishkan.
Additionally, in the Ralbag cited, he proceeds those statements by saying that we do not know the golden ark thicknesses [at all].
ועובי ארונות הזהב לא נודע
The Living Torah (ch. 38) breaks down the gold distributions in the Mishkan:
Since a talent is 3000 shekels, this was a total of 87,730 shekels,
1/7 shekel for each person counted. It was a total of 4386 1/2 pounds
of gold. According to Saadia Gaon (quoted in Ibn Ezra, Perush
- 1/2 talent was used to plate each of the 48 beams of the Tabernacle, accounting for 24 talents.
- Another 2 talents was used for the four pillars holding the cloth partition.
- 1 talent for the menorah.
- 1 talent for the ark and its cover.
- 1 talent for the table and golden altar.
This accounts for the 29 talents. The additional 730 shekels (36.5
pounds) were used for such items as the fasteners and clothing.
[Since 1/2 talent was used for each pillar, which was 10 cubits high,
and 1 1/2 cubits wide, it can easily be calculated that the thickness
of the gold on the pillars was approximately 1/100 inch. If it is
assumed that the same thickness was used all around the incense altar,
it can be calculated that the top of the altar had a gold layer around
1/4 inch thick.]
Earlier, too, he presented (25:17) similar contradictory information you find yourself with regarding the ark cover:
3' 9' x 2' 3', like the dimensions of the ark (Exodus 25:10). The
Talmud states that the ark cover was one handbreadth (3 inches) thick
(Sukkah 5a). It can easily be calculated that if it were solid gold,
it would weigh (without the cherubs), some 2500 pounds, or 17 talents
(see note on Exodus 25:39). Some sources thus state that the ark cover
was considerably thinner (Tur), since we find that the ark had to be
light enough to be carried easily (Baaley Tosafoth on Exodus 25:11).
One source states that the ark-cover weighed one talent (150 pounds),
just like the menorah (Exodus 25:39; Saadia Gaon, quoted in Ibn Ezra
on Exodus 38:24). The ark cover would therefore have been around 3/16
inch thick, or, if the cherubs are taken into account, more likely
around 1/8 inch thick. It may have been made like an inverted open
box, so that its sides were one handbreadth thick on the outside.
The most lenient opinion cited would still be too large to fall under the 1 talent prescribed for both the ark and ark cover.
An answer can be found with the Ibn Ezra cited, expounded and explained by 'Mishenh Lezra'. An additional problem he cites is regarding the leftover 730 shekels - how can that small amount of gold, a little more than 1/5 talent, provide for all rest of the odds and ends in the Mishkan (from clothe items to structural items, see link for full list)?
He answers that in counting the total amounts of gold and silver, the Torah did not go into specifics regarding what the gold was used for because the gold was donated. As apposed to the silver which was mandatory. The numbers of the gold were not as relevant, especially compared to the silver which doubled as a census device.
It thus comes out that we cannot necessarily know what the measurements of the gold were, and, if desired the Ibn Ezra presents an opinion the fits the ark into one talent, which removes the question at hand who places it at many talents.
[It is also probably likely that more attempted calulations of other gold parts of the Mishkan may lead to a number exceeding 29 talents, as well.]