How did you adjust the TV cable? It is a quite sensitive adjustment, and as little as 1/8" makes quite a difference. Mine took quite a while to get right. A good starting point is to open the throttle all the way and then adjust all the slack out of the cable. This is usually too much tension on the cable, but it is a safe starting point.
Also, don't get me wrong. The 700R can take a lot of punishment, but it was never designed in stock form for something with the power of a 426 Hemi. The most powerful engine put in front of a stock 700R was the 454 found in the
84 and newer trucks. I believe the SS454 would have been the most powerful one, but I have no idea of the HP or torque that engine puts out. It's not even in the same class as a stock street Hemi, let alone a Hemi that has been modified.If the trans is slipping in all gears, then the pump is probably not doing it's job, or the TV cable still isn't right. The older 700R pumps were prone to cracking the ring that retains the pump vanes. If this happens, then your pump will lose most of its pressure, which in turn will cause less apply pressure to the clutches and band.
If it is slipping in 2nd and 4th, then the band is gone or the apply servo isn't applying enough pressure to hold it. This could be because of a bad servo seal, or just the fact that you are overpowering it. There are aftermarket servos available, in addition to the Vette one. The nice thing here is that they can be changed from the outside of the case, as long as you can access the servo cover. But like I said, the bigger the piston here, the slower it will shift into both 2nd and 4th gears.
If it is slipping in 3rd and 4th only, then it is more than likely the
3-4 clutch pack, or the apply piston seal. The 3-4 clutch failure is quite common due to the lack of cooling of these clutches. This is because of the transmission design. The clutches are buried inside everything else and are the last thing to get lube, even though they are the ones doing the high load work. The mods for the 3-4 clutch involve drilling extra oiling holes in the clutch drum, as well as upgrading to 2 extra clutch plates and steels in the clutch pack.BTW, whatever that Hemi is in with 4.56 gears and a 3.06 first gear has got to have one hell of a holeshot. I hope your friend gets this sorted out.
Also one other note. Painless makes a wiring kit for the converter lockup on non computer vehicles with this transmission. It is basically a new solenoid that goes in the valve body, along with a brake switch and a vacuum switch. The kit allows the converter to lockup in 4th gear only, and it automatically unlocks when the vacuum drops below 12" (I believe) and the brake pedal is not depressed. This is the kit I used in my S10 and it works great.
Chris