Wow, nice car!  The 70-71 fastbacks are super cool cars, one of my favourites too.  

As for the torque boxes, I think Sherman used to repop them but they are no longer in production.  I quick search on google found passenger side NOS repops on eBay but no driver side.

https://www.ebay.ca/itm/127035069290?chn=ps&mkevt=1&mkcid=28&google_free_listing_action=view_item&srsltid=AfmBOorkaemS13jJG9Y_Pd3U8gB9kkdufLy_qyiAQ-U6CFmQOnCqMlKrVYk" rel="nofollow - https://www.ebay.ca/itm/127035069290?chn=ps&mkevt=1&mkcid=28&google_free_listing_action=view_item&srsltid=AfmBOorkaemS13jJG9Y_Pd3U8gB9kkdufLy_qyiAQ-U6CFmQOnCqMlKrVYk

One of our members, JRanger, who has a video on torque box repair:

Here is his project thread which may have more info:

https://forum.grantorinosport.org/patches-71-torino-gt_topic20621.html" rel="nofollow - https://forum.grantorinosport.org/patches-71-torino-gt_topic20621.html

so H code vin? could've also been a 351W-2V so you may want to investigate the block as well, casting & date codes are near the starter. 

here's the low-down on the various 4V Cleveland engines offered over the span of production

the easiest, most accurate way IMO to determine what heads you have is by the date code found on an intake runner in the valve spring area. along with this chart compiled by members of the Cleveland's Forever forum. click on this link then click on the pdf to open the 351C 4V Cylinder Head Repository 

https://www.tapatalk.com/groups/351c/cylinder-head-repository-home-of-the-dot-no-dot-t4793.html" rel="nofollow - https://www.tapatalk.com/groups/351c/cylinder-head-repository-home-of-the-dot-no-dot-t4793.html

there'll also be a casting date code on the intake but if it's a square bore carb flange it's M code, spreadbore it conforms to Q code / Cobra-Jet. there are issues with both types, the square bore manifold has small throttle bores and will not accept a big carb. the spreadbore manifold only accepts a 4300-D carb or requires a specific Trans-Dapt 2199 adapter plate. i wouldn't sweat the Cobra-Jet thing, some people get way to worked up about it. FWIW the factory CJ is a low compression smog dog, but it can be woken up!

I agree with you that it's just good to have the engine platform. It has good potential, the intake is useless if it's a stock based design for a 4V head. The 4V heads need much better than stock intakes, which there are almost none to choose from. The RPM Air Gap is the best hands down for anything but a serious high rpm 351C.

My last Ranchero was a Q code, but as you can see by the chart, that means open chamber and low compression. That's fixable with closed chamber heads, but expensive of course like everything. This engine I have was rebuilt with closed chamber heads, and it runs okay. But it is mediocre because of a mediocre camshaft and a Performer intake. So the rpm band is the stock lower level, maybe a 5k shift point.

The 4V heads don't become efficient until 3000rpm or so. That's why some now really like the 400's, those have 2V heads which are much more efficient at street rpm levels.

Take your time fixing your car to get it to run well and have good brakes etc. Make it safe before worrying about bigger upgrades.

Sure. This picture is from the wheel well perspective. The support piece goes underneath the apron and the tray bolts (2) will thread in from the top and one will thread in from the side bracket. Based on the repair on yours, it looks like you'll need to re drill the holes in the apron.

After bolting the tray in place, I had a friend put a few spot welds for extra support.