Gerald, welcome back.

Gotta go to the basics. Need to know stuff. Start with the stock system. Kv rating of the motor, volt rating too, gearbox ratio, prop size, amp rating of the esc, old pack cell count. And you absolutely need a Watt meter.

The older stuff with brushed motors, you have to be careful of the cell count when using lipos on them. Most won't take a 3S lipo for very long. A 2S will drop the performance slightly but will hold that level longer then the old Nimah/nicads did. I'm guessing the motor is a fairly high kv rating, hence the gearbox vs direct drive. Probably a 6 or 7 volt motor.

Lots of changes in 5 years in electrics, stuffs a lot better and cheaper too. With all the China stuff out there, you can get a motor,esc and pack for way under a hundred bucks now that would make your stang run like it was supercharged. And do it direct drive and at lighter weight then with those old style cells.

And plenty of calc programs out there that allow you to play with motor/pack/esc/prop/airframe changes. Takes a lot of the guesswork out of setting up a plane, and a lot less costly doing it in the calc program then in real life. MotoCalc is my favorite, it has a 30 day free trial. Give it a look see, doesn't take long to get the hang of it.
www.motocalc.com

Oh and your packs are good for a maximum of 12 amp draw. Rule of thumb is to run a lipo at 80% of it's rating to get the best life out of it, so you're looking at just under 10 amp draw for them.