If you are not technically adept then leave it alone. The transmission is probably different. That said, a car that old is not hard to swap engines in. They don’t have all the confusing electronics and computer controls.
It seems like a shame to move one of the most sought-after Ford small block V8s for a rather ordinary 6. If your Mustang came with an automatic, you might want to check if the FMX automatic is compatible with the low-protein 6.
The 351 Cleveland is sought after by collectors. In the long run it may be better to have it overhauled as an investment.
Some one put the work into transplanting the 351 into your 67 that never came with Cleveland’s. To do this transplant right required front springs, spindles, brakes, cooling system, motor mounts, trans, cross-member and trans mount, drive shaft and matching yokes, rear axle and leaf springs.
so reverse the process.