[2blu442]

My experieces has been if your keeping the metal engine perches from the 350 then use 350 rubber mounts. If you change to the 455 metal perches then use the 455 rubber mounts. The bottom of the engines are the same so they both fit, but where the difference comes in is the exhaust manifolds. I mixed the perch/rubber mounts on a car with headers and the headers wouldn't fit. I had used this engine with these headers in a different car and they fit fine, so it took me a while to figure out what I did wrong! I've made the swap from TH350 to TH400 several times and always had to change the driveline to a shorter one. Check your trannies again to see what they are. They should have both been short tailshaft versions for the A-body car, but if someone put a long tailshaft TH350 in that might make them closer in length, I don't know, I've never compared the TH350 long to the TH400 short tailshafts.