There was a frame change for Sweptlines around 1968.

My '64Dog has four engine mounts and the transmission hangs off the 70 lb cast iron bellhousing. The forward motor mounts are the same as later frames, but there is a beefy crossmember in place right back of the engine. The bellhousing has two mounting ears that attach to mounts on this crossmember. This crossmsmber is a major pain in the rear, because it is permanently attached and the engine/transmission can't be slipped in from the front. The crossmember and the firewall don't have enough vertical space to fit the transmission through. That's with the NP420.
I've seen here a frame from an auto trans early model and the big crossmember has a bunch cut out of it with an attachment to support the rear of the transmission at the traditional transmission mount.

My '71 donor truck has the later model frame, and the rear crossmember isn't nearly as beefy and bolts to the lower flanges of the frame rails.

From the beginning up to the very early 1965 models the longbed had a 122" rather than a 128" wheelbase.

So, even though the trucks look the same, there are differences that can eat your lunch on a build.

R.