Boxing the frame may be overkill and overthinking things. There have been several guys on Inliners making more HP than you'll be making, both N/A and turbocharged and racing their 6 cylinders in these old trucks like this one and haven't needed to box their frame rails, so you probably wont either since you're expecting relatively low HP comparatively speaking.
As posted above, for your needs and power level its overkill and not needed. If you are still concerned, just buy an Art Morrison frame with modern A-arms and suspension and be done. Some things just aren't DIY friendly. Just because you take a class and buy the tools doesn't mean your qualified to use them, it usually just gives you a false sense of accomplishment. It still takes years of experience along with that to become competent.
Art Morrison Trucks
