Have you tried these guys?
Roko Overland Build Vehicles
– Roko Vehicles LLC
Very knowledgeable, Framingham, now KO Ineos but they still have a garage somewhere nearby and are associated with Wellesley Toyota.
Talked to a few people who were happy with their work.
Also Main Line Overland in Bow, NH
Main Line Overland: Auto, 4x4 Specialist for Cars, Jeeps, Trucks, SUVs
Maybe both are more medium sized shops, I'd be interested also if anyone has smaller shop recommendations in NE area.