This is just a wild guess and hopefully someone else here knows more. The timing chain sprocket is very different between the two engines. Part of it is associated with the cam position sensor and ignition timing. You might have to get a K5 ECM or, if you're lucky, install the K3 sprocket on your K5 engine. Don't blame me if you swap sprockets and it blows up.
P.S. I'm guessing that the K5 ECM uses different connectors than K3. If so, you're really going to need to swap harnesses too in order to swap ECMs.
Last edited by billv; 11-09-2019 at 01:08 PM.