Ken,
I've not seen how the 49 column originally mounted, but if the 77 column uses a girdle type mounting bracket that wraps around the column then I can picture the problem.
If the tap doesn't work or doesn't give you enough strength you might try what GM did on my column. My 64 Camino (the 92 S10 column) has four nuts welded to it for the bracket to attach to. They are thick enough to give the threads of the mounting bolts some bite. Since you have to build an adaptor bracket you could adjust for the space taken by the nuts.
Mark
