i have the same problem with the bottom tensioner. the hole for it is allready there however no surfast to sit on, after reading the liquied metal idea i went and purchased some selleys stuff.
however it says will only handle 120C of heat. would the constant spinning of the tensioner create a friction heat of excess of that is what i am wondering, considering if it did fail and the tensioner let the belt slip off it would cause some rather major damage.
or would getting a cast iron welder to fill it up then smooth over be a better option.
or just buy the right block in the first place, hits self over back off head