I read an article years ago about race prepping chassis. They welded all of the unibody seams and blended them with a grinder. This is probably overkill for a street car.
Mustang Plus is a good option. They sell a complete strengthening kit that includes torque boxes and additional steel under the chassis. I'd be afraid of it getting really heavy however. They originally designed it for their Ronster kit but also used it on their Colt351 project car.
Your best bet for street use are sub frames. IMHO
Maier Racing makes the best designed subs for vintage Mustangs. I prefer their design over what is offered by everyone else, Including the
TCP units. The TPC has weld in subs and a bolt in cross member. Except for adding a few more pounds, I don't see the extra cross member helping that much since they are not connecting to any additional mount points to the chassis. It does provide a nice mount point for a torque arm but that is about it.
Cobra Automotive also sells a heavy duty cross member that ties in the front
lower control arms. I know they have them for '65 & '66. I'm not sure about your application.
Export Brace and Monte Carlo bar is a no-brainer. Definitely worth the money. You can go with the old style which everybody has, or the newer style like that provided by TCP. I like what
Street or Track has to offer for the monte carlo bar. It's simple, well designed, and cost effective. It is also super convenient when you need access to your distributor. You can remove the cross bar in about a minute.
I went with the chromed old-style monte carlo and export brace. I may upgrade at a later date when $$$ is a little easier to come by but all that chrome will sure look nice.
$0.02