[BR3W CITY]

I was about to say "when its warm, and in the garage"...but that clearly isn't helping.

How much power are you actually planning on making though? "big cube" small block would lead me to think in the mid 400ci, but that could be anywhere from 350hp to north of 1000.

Since your not installing a motor that itself weighs more, boxing your frame might not have the effect you really want. All of the additional metal will add weight to the vehicle, effectively counteracting some of the new power you plan to add. The strength gains also may not be in the places you'd hope.

Boxing the frame for a heavy motor (huge bbc, diesel, something else), or in something that will be heavily loaded can work. Boxing can also help resist the forces trying to "fold" the frame, for example from being jumped or from doing a wheelie and coming down hard....but it isn't as effective in terms of resisting frame twist.

To combat that, its all about finding a way brace/connect parts of the frame so that the force being applied to it can be transferred in a manageable way, or counteracted by another force. If you were to take most of the rungs out of a ladder, you could pick up on side and watch the whole thing twist...the truck works the same way.

Tubular cross bracing, replacing or bracing crossmembers, rollcages all have this effect. I'd spend the time and the effort on a tube back half for example, which is not only lighter, but stiffer (but does not have the same load carrying capacity). You won't see a ton of fully boxed frames going down the strip.