Linear constraints are more complex than box constraints (which are very simple forms of linear constraints), so I do not see why you would want that conversion. Box constraints are also very easy to handle: if you have a favourite search direction you can use a line search methodology based on that and use the very simple Euclidean projections to produce a very fast (per iteration) methodology. As an example (although it may be slow), simply use the steepest descent direction, and use the Armijo step length rule based on the starting point and the result you get by taking a step along the negative gradient, followed be the very simple Euclidian projection onto the feasible set. (The last operation is very fast, since all you need to do is to compare each coordinate's value and if one value is too small or too large you simply bring it back to being feasible.)
You are facing a simple Non lineal Programming task. For it solution you can use any of the numeric Non linear Programming methods, by example, the Localization of the extreme of the function of one variable, if it is a single independent variable problem or the Exploration if it is a problem of more than an independent variable. These methods you can find in any book devoted to the numeric optimization methods..
Completing the previous answer: I have told about the Exploration in a Net of Variables method, that requires the inferior and superior bounds determination to each variable. The objective function could have any form, including quadratic functions as a particular case.