I am a proponent of re-using old PSU units where possible. No point in getting rid of something that works.
Keeping in mind that the older it is, the less reliable it will be. This is compounded by both it's rating and build quality that it started from. Consider that when PSU go (in a bad way) they have a tendency to take things with them. You have to decide whether the value of this builds replacement is a good gamble against the price of and the perceived durability of the unit you are going to redeploy.
In my own case, when I recycle or "cheap out" for a budget build I like to keep the draw at no more than ~50-60% of the peak rating of the PSU, where possible.
Since you have the 400w psu, go ahead and try it.
You are unlikely to overload the psu and that is where the real danger is.
Do you now have parts, or is this a prospective build?
What is the main use for this pc?
List the make/model of your old pc and the new.
I do not think I would build a new pc today using a 2200g
You may have better options.