Do you have the transmission mount on your truck?

The 1954 and later rear mounts and frame cross-member must be a little stronger set-up because they allow the transmission mount to be removed.

Are you drag racing? If not, my guess is that you have a problem not related to the 261.

There are people on the oldGMCtrucks site with experience mounting the 302 in a variety of trucks. They'd have experience and advice on rear mounts.