standard axle is good for up to 100BHP
not sure on the atlas axle although i do seem to remember hearing that somewhere before so there may be some truth in it.
Your alternatives are:
manta A - expect to pay 50-75 quid fo rone without the torque tube and 100-150 for one with the torque tube. Having said that there are bargains out there to be had. to fit this you will need to turret the rear shocks using manta GTE shocks (billies as standard

)
manta B - expect to pay about 20-50 quid at a scrappy, there are plenty about, or just buy a dead manta and strip it. You will again need to turret the shocks, but you can get them off the manta you are getting the axle off. You will however, also need to fit wider arches as the axle is slightly longer than the chevette standard
for the manta axles, you will need to change the mounting brackets to the other sid eof the axle with some creative welding, but its an easy job apparently, I think you also need to put the panhard rod onto the opposite side as the mountings will nowbe sat where the panhard rod should be.
chevette HS axle, erm, rare as rocking horse poo. Again I think you need the wider arches, be prepared to part with an obscene amount of money and three kidneys.
some prop work may be necessary for each of these axles, just to get the right splines, or yuo may be able to change it over at the Universal joint?