Use your 1953 GMC clutch housing and rear motor-mounts with the 261.

If the 261's flywheel to crankshaft bolt-pattern matches the pattern on the 228, you could use either flywheel and the clutch assembly that goes with that flywheel.

If both clutch assemblies have the same clutch size and pressure-pate bolt patterns, you cold use either assembly.

Someone else will comment on the front motor mount. The lengths of the blocks are different.

You might have to "fiddle" with the throttle rods.