Its not the brake horse power that breaks them but the torque, and with your engine you only going to have a max of 140ft/lb

that said it needs grip to break it, Newtons laws of opposite and equal.
If you can only put down a certain amount of torque and its lower than the breaking point then its safe.

Of course if its grippy and you have some really high ratios then you may come into problems.
(A)
150 ft/lb x 1st gear 2.9:1 = 435 ft/lb x the CWP ratio 4.1:1 = 1783.5 ft/lb.
(B)
150 ft/lb x 1st gear 2.4:1 = 360 ft/lb x the CWP ratio 3.54:1 =1274.4ft/lb.

500 ft/lb diference

The reason why some cossied powered Escorts dont break the english axles is because they cant put the power down, they can on the rollers but thats done in 4th with a gentle drive up the revs, and they are strapped down. The Torque they supply is far in excess of what the tyres can grip, a spinning wheel and you have no grip.

(C)
400 ft/lb x 1st gear 2.9:1 = 1160 ft/lb x the CWP ratio 4.1:1 = 4756 ft/lb
(D)
400 ft/lb x 4th gear 1:1 = 400 ft/lb x the CWP ratio 4.1:1 = 1640

3000 ft/lb difference

so you see in 4th when the power reading are done the torque is a lot less.
and if you can spin the wheels with 170bhp x-flow you spin them with the cossie at the same rate, and for longer,

You have a light low grip car, thats easy to change the box mounts on, so go with that box you have first and see what happens, i doubt you will break it on street tyres.