Have you tried wallace calculators?
On my old 451 duster, The new owner installed 4.56 gears & kept the 28" tire & it trapped 71-7200, The engine was making over 500 FWHP, but you need to calculate converter slippage also. Its really not HP, Its the gear/tire/converter slippage, you can trap a 300 HP engine with the right gear/tire. Not that it would pull that high but its all in the math. Heck i used to shift my mild 340 at 7500 with stock pistons, but it had triple valve springs & a helthy crane cam, so with the right gear/tire i could have trapped it too 7600 & it was only a 330 HP engine at best.