Metallic clanking from rear end when car is in motion suggests broken part in drive shaft or differential. My mechanic needs guidance or someone to take over.
Likely a broken pinion bearing or disintegrated Babbitt thrust washers in the rear. If get the MTFCA rear end how to manual it should have all the answers.
Have you contacted the Old Colony Model T Club on the South Shore? We have many members who may be able to help (many in Kingston, Middleboro area).