Just get the galv chassis, 99% of it will be protected perfectly, the small area you will weld will be insignificant, just treat it well after, waxoil etc etc, as for the fumes, yes they are not very clean but having done lots of galv welding years ago when people didn't realise the health implications i have to say I am still very well indeed. To be honest, a half hour welding will do all you engine / gearbox mounts with ease, no health probs there, I've done 30 yrs plus, and still going , you choose ( just chooose galv for the rest of the chassis sake ! ).