Frame not suitable for 70" fixed gear?

No. Think about where the loads of the frame are coming from. Ok, there's your weight and the vertical loading coming through the wheel/seatpost/handlebars. But there's also a twisting moment coming from your pedals which primarily acts on the bottom bracket. (you've also got your handlebars giving the same kind of moment). This bottom bracket load comes about because you're applying force to the pedals, and this force is at a distance sideways from the bottom bracket. This causes the bottom bracket to twist, and this resistance to twisting (bottom bracket stiffness) is provided by the down and seat-tubes, and also the chainstay.

More force/torque, more frame load. Big strong sprinters need frames with beefed up bottom brackets and it's not unheard of for frames to fail at the bottom bracket area.

I see what you mean.

TBH I was thinking about my last post, and realised that the frame is connected to the drive both before and after the gearing. Especially if, like you say, you consider handlebars etc.

Like I said its monday, and the brain is just starting up for the week.