If you haven't cut too micxh yet, try a Silver Fox DMS conversion, which has the correct window spacing for the DTS in 2 pieces plus seating. It is the old TPM casting. I'm just lining out my 121 DTS conversion.
Don't forget that when built, the 121 trailers did not have corridor connections, these were added mid to late 80's to allow them to operate with other dmu class power cars as hybrid sets. Info from this forum.