change \sqrt symbol

\DeclareMathRadical is a wrapper around the \radical TeX primitive and in classical TeX, that takes a character position for the sqrt sign and takes the rule thickness for the over bar from the height of that character. If you look at font tables for TeX fonts the sqrt sign character hangs almost entirely below the baseline so that the rule gets a reasonable thickness. The \radical primitive then raises it while typesetting.

This a major reason why TeX fonts are not easily usable by non-TeX systems and the other way round.

You could set up a virtual font in which this character is lowered (someone may have done that already, I'm not sure) but if it is just for occasional use for normal sized arguments it is simpler not to use the \radical mechanism at all, simply use the character as a normal character and use \overline over the argument with some spacing adjustments to make things line up.

It is not exactly an answer, but I can see that you are interested in modifying the radical sign. There is a very interesting attempt to this problem: http://www.gust.org.pl/bachotex/2011/program (Marek Ryćko, A Radical Approach to the Radical Sign).