Back in the day (circa 1999-2000) I did the 5.0 swap to my first 740, using Converse's kit. I had a stock 5.0 with stock clutch, and used the cable setup (swapping FROM hydraulic to cable operation in my 16V wagon), since that was what Converse advised. Used the 5.0 cable mated to the Volvo pedal per his instructions.
It worked fine. Maybe a little heavier than a Volvo pedal but not by much. Certainly not enough to object too, and if anything it was lighter than the Mustangs I drove with the same setup. The matchup of the pedal lever ratio to the fork lever ratio was right on, not a problem with the physical compatibility. The Mustang mechanics you talked to are confused on that. I think you're looking at another issue, maybe grease dried up on the release bearing sleeve/fork pivot, or clutch is tired, or someone put in a crappy upgrade clutch. I'd be taking the trans off and just throw a known good new clutch kit at it first, before going crazy with other ideas or breaking something in the pedal box/firewall/pedal/etc from the high effort.
You shouldn't have to reinvent the wheel and change to a hydraulic setup -- no doubt it would be nice, but that is a lot of work compared to just making the cable setup you already have work better, which it can. Converse built a lot of those setups and sold even more, and clutch effort wasn't a common complaint. Some had heavily modified (blown or turbocharged) 5.0's too, so he must have been using heavier clutches sometimes too. A higher capacity clutch for a modified engine doesn't necessarily have to mean insane pedal effort or grabby engagement. The best designed ones add torque capacity without multiplying effort much beyond stock. Look for something from Centerforce, South Bend, Clutchmasters, Clutchnet, or the other respected names for a product that'll perform well but not destroy your leg and firewall, or, if the engine mods aren't too wild and you don't plan to drag race every day, just put in a stock kit, those will work fine for most reasonable situations. The cheap uprated clutch options out there will hold the torque but suck to operate.
The only issue I remember with my 5.0 T5/cable setup was the threaded adjustment on the cable end at the bellhousing would tighten up over time from vibration and I would lose the free pedal. Had to add a jam nut to stop it from doing that.