That looks exactly like my 400! I spit my driveshaft out at the finish line , it had a slight vibration for .5 seconds bere leaving!. However I think the trans may have been cracked before. I had a leak and I could never find it , the general area was where the cooling lines go in. I also cracked a Th350 before I switched to a 400. I too run poly engine and trans mounts and I'd say my body mounts are in poor condition. 70 Monte , 3850lbs w driver , 530 hp 383 , 4000 flash. If it makes you feel better I'm on my third trans. TH350 broke case , TH350 broke sprag , TH400 broke case.