Yes it can be post cured - however, without support there is a risk of sagging or warping, particularly if it is a large and heavy part.
Thermal curing is simply a way of speeding up the chemical hardening process, which in the case of typical epoxies may normally take place over several days, even weeks. The second benefit of heating the material is the removal of dissolved gasses in the plastic (outgassing)
There is no need to cure at 250F - any temperature above ambient will hasten the curing process, so I would suggest soaking at say 125F for 12 hours instead of 250F for 4 hours.
Warping and distortions are always a possibility but in my experience if the temperature is managed and some support is supplied, (e.g. a sand bed) any distortions are imperceptible, at least in my applications.
This is a very good reference for those interested in the process: Masterbond.com