Will a Stock 489 handle 700hp?

My guess is he's adding nitrous or boost. Most power adder cars don't need as much gear as N/A cars. Same deal with converters, a converter from a boosted combo will be tight in a N/A deal unless it's big inch, monster torque.

Several years ago I was at I-57 drag strip and saw a nice 69 Dart street car with 275 radials and GT hood. Watching him make a pass it didn't 60' very well but it was flying on the big end, it ran 6.0's (1/8) at some insane mph. When I checked it out I found it had a 588" (IIRC) wedge and was pulling 3.23 gears. That explained the soft leave and huge mph.

To the OP, call Strange and talk to them about a S60, buy once, cry once. I bought mine directly from them and other than giving them my CC number it was painless.