Get a roller 351w.  Middle '90's F150's had them.  Then you an build as much power as you want.  And the best part is that 351w's came stock in 69's.  No fitment issues.

Your subject title mentioned Explorer & Mustang. I'm guessing by this you are referencing the possibility of a 5.0L EFI donor from either of these (?).

The Fox EFI engine will have a TFI (Thick Film) Ignition distributor. The Explorer 5.0L engine has an EDIS (Electronic Distributorless Ignition System), --no distributor. The Explorer 5.0L will require an EDIS module and specific wiring harness to make it work. The 5.0L Explorer engine will also have the 'P' heads with straight plugs compared to the Fox 5.0's angled plugs. That would be important come header installation time.

A fuel injected 5.8L (351W) engine will have the potential to make a lot more power than a 5.0L engine. A truck 5.8L could be the basis for the swap but not with the tall, stock truck EFI induction. The only stock 5.8L upper & lower intakes (GT-40)  that would fit under the hood of an early Mustang would have to come from a '95 5.8L Cobra (pretty rare), or from a '92-'95 5.8L Ford F-150 Lightning pickup. --if you could locate a complete 5.8L Lightning engine, that would be a really good swap. Outside of those two stock 5.8L induction possiblities is the aftermarket for obtaining the EFI induction pieces. There's also the option to use a carbereted-style 351W intake fitted with fuel injectors and a throttle body. Taking this approach though will have a dramatic reduction in the engines low-end torque curve. It would be primarily something that would be designed for track (drag strip) use where the engine is going to be in the very high RPM band of operation. --long EFI intake runners are for torque production. Short EFI intake runners are for high RPM use.

The 351C was out of production long before the advent of Ford factory fuel injection. It is possible to install 5.0L/5.8L-style EFI upper/lower intakes on a 351C to fuel inject it but, it require specially machined adaptive components so it's not a simple and direct bolt-on. --search for "351C fuel injected Pantera," to find info on how a guy in Sweden injected his Pantera's 351C.

You went EFI on your new Engine MS?    How do you like it?