Conversion the other way has been done to do away with the t-bar, but I don't recollect anyone wanting a t-bar, only not wanting it! It may be possible, but a better option would be to buy or trade for an SB1 frame as Ranger suggested and fit the M158 barrel to it, there are some available in the H&R classifieds on GBO.
My best guess at conversion is the frame will have to be milled to make room for the lifter striker assembly.
Tim