been looking for a 496 mag HO but came across a merc 8.1 HO horizon kind of local to me, I understand the horizon bolts on to a transmission and the 496 bolts on to a Bravo, I do have a bell housing , coupler and starter so thats not a issue, I checked on mechanical parts, long block is the same, cam is same, both have mechanical shift and throttle controls, can I make this work? I guess recalibrating might be needed, will the engine harness from the 8.1 Horizon work? has anyone got any experience with this?
I do not have a 496 to switch parts over,

I believe the harness is the same but the calibration is not. I seem to recall reading that the Horizon versions (sold in straight inboard or V-drive configurations) had a lower rev limit. Everything else should swap over other than the parts you mentioned above.