Hi Jim,

I wouldn't be concerned with the weight 76KG for the gantry is not excessive. I would always take a slightly slower heavy gantry over a skinny fast one anyday. Mass can be your friend if matched with right motors and screws.
Regards the 16mm screws.! . . . Depends is the only answer I would give without knowing the full dimensions. I'm pritty sure they will handle the mass/force's etc but length will determine if they are suitable without whipping. . . Give more info if you can.

Don't know those motors.! Are they Nema23 or 34's. . . . Personally I prefer 34's for larger machines with weighty gantry's. They don't quite spin as fast as 23's but the torque remains higher up the power curve, giving strong torque without much drop-off as the rpm's rise then drops off the clifff really fast around 8-900rpm.
23's tend to drop gradual as the rpm's rise but will carry lower torque to higher rpm's 1000-1200rpm + is not unusual.

Again without knowing the full details it's hard to give a definate "Yes" or "no" but @4nm I think you'll be ok for most single ballscrew setup's. Ball screws are very effiecent so allow less powerfull motors than other linear drive system's IE:R&P etc.