My '88 351w used a flat tappet not a roller. To switch you will have to use a retro fit kit. You can't use a big cam with the computer without programming so no point in going to the roller cam. Stay with the factor replacement parts and don't buy high performance parts because it is pointless and will usually cost a bit more and require modifications. If you want to get crazier with it then you will need to look at some mustang forums since that was where they did a lot of modifications on those computer systems.

Swap the cam and exhaust first and see how that does. If you have to swap the valve springs you'll be in for a treat. If you don't mind the work there is no point in not refreshing the top end with new springs and rocker arms (1.7 ratio is a possible upgrade). Push rods too. That cam you listed should have a recommended parts list on what matches it. If you have questions the cam manufacturer is the best place to ask. (I've used comp cams and crane)

Don't go with an intake yet. That may require programming and injectors.

Heads would be your biggest bang for the buck but will definitely require programming and injectors.

Last but not least is if you need to rebuild the motor is to stroke it to a 377 or so for extra torque and hp.

There are a few guys here that have built the motors up in their vans with the older OBD2 system but they usually have drivability quirks. If money is no object, and using the C6 with no computer control, a programmable Holley type fuel injection system would work great.