You could generate a C-terminal forward primer with construct specific sequence from the stop codon onwards with a 5' linker to include the both Myc-tag and (if required) specfic protease cleavage site (see, for instance 'epitope tag removal' proteases on the sigma website) and a reverse primer complementary to sequence immediately prior to the stop codon, amplify the whole plasmis with a high fidelity polymerase and then ligate.