Not hard to 400A out of that setup. VERY hard to get 400 amps for more than a few minutes without melting it down. Alternators are usually 60-70% efficient. So if it is putting out 20KW, it is generating 13KW in heat internally. Where's it gonna go?
Even a reliable brand (I am using a Delco 28si) will seriously derate as soon as it warms up. My alternator is rated at 160A but will only do about 110 continuously and stay within temp specs. I do have an Electrodyne on the sailboat which will do a continuous 280A @ 24V, but it is a huge piece of gear with external diodes, two large fans, and is driven by a driveshaft.