Swapping dynaflow trrans on 66 401 engine with 400 trans

I’m restoring a 66 Buick wildcat convertible and the original engine was a 401 nailhead with matching number to the vin. The engine had a blown rod thru the block which I was going to have the hole welded up and the cylinder re-sleeved. But when I had the block x-rayed we found the oil tube damaged and fine hair line cracks in the cylinder walls. So I scrapped the block and purchased another 1966 engine from a wildcat.
The problem is the original engine had a 400 trans and the replacement had a dynaflow trans. I was told to change the flex plate
Which I did but after checking the mounting to the engine the 400 was ½” to short. Is there an adaptor ½” thick available for this or am I doing something wrong? I double checked the mounting of the torque converter and it seems right.
Any help will be appreciated.