There are several suppliers of steel 4-bolt caps. But, unless you plan on using this motor in a SERIOUS Drag car, we're talking Funny Car/ Altered fuel burners. The costs are just not worth it. A set on main cap studs will give you all the strength needed.

Aside from the cost of the caps. You will have labor costs in the fitting/milling of the caps and the final line bore/hone of block.