Is there any way to run the Thunderbolt IV with the timing locked out with an MSD box? Thanks, Alex

Not sure there is a good reason to switch ignitions. If you use the Merc V-6 box with the TB-IV distributor, it is a pretty bullet proof set up up to 6000 RPM. The V-6 box will provide similar benefits to locking out the advance. It will also give you stable timing at idle....not with MSD.
You can interface an MSD rev limiter if you want a rev control feature. You won't find a lot of support for the MSD ignition on OSO.

Thanks... But does anyone know if you can trigger the 6m box with the pick up in the TB IV? I was told you can hook it up to the Mag pick up, but I'm not sure how that would work being a Hall effect pick up in the distributor. It's a 1300hp tall deck blower motor that likes lots of initial timing ( more then with the v6 module) and I want to keep the merc Distributor. Thanks again, Alex

Absolutely!..Go to MSDignition.com ..find the model number of the box you are using..ie: 6460 then click on instructions. type in the model number 6460..you will get the install instructions for that box.. follow the wire diagram for the mercruiser setup.. and you are all set!..the MSD box is the sparker but your TB-IV is the rev limit and timing..