Between the two, I wonder which would be the most beneficial for stiffening, if I could just pick one? The longitudinals/frame rail channels or chassis stiffening kit?

Do you mean the clam shells VS the in the cabin stiff kit?

Not owning either of the above, the clamshells are more for rust repair then stiffening. The in the cabin kit is where its at - but you HAVE to be careful welding it in or you will twist the chassis.

Zach

The Engman kit is very stealthy and leaves the car looking original. If you follow his instructions and do some searches on this site for other guys' experiences and recommendations, you shouldn't have trouble with the dreaded twist. It's about taking your time, and keeping the concentrated heat to a minimum.

outer long overlays will give you the most overall support in the long run.

Rich

Mike at Restoration Design actually told me that his #346 clamshell was for stiffening the chassis and not for rust repair. It slips over the stock long. His #340 and #341 were meant for the rust repair.

The outer longs are for stiffening and require the under lying sheet metal to be fixed and in stock shape, jack post included, to add this layer on top.

The benefits are several.
First, it covers the whole long and extends up the angled section in the rear to attach up to the rear suspension console.

Second, it does not require you remove your interior and get all that work done.

Third, you have the ability to straighten the car to some extent due to the the complex design of the stamping...

While the inner long kit has merit, it only covers up to the rear firewall and that can in some instances not solve the flex problem.

Look at the inner long just behind the firewall and you will see the heater tube that is cut into the long. At that point, all the load is transfered to the outer long...and its at a joint in the outer long.
The overlay has no joint there so it really adds quite a bit more stiffness to the car.

On the inner longs with these older cars there is a specific area that develops a crack over time that you should address.
The crack is straight down from the windshield and the crack develops on the vertical face of the long starting at the bottom.
This happens to be where a majority of the stress from the car resides when it starts to flex and thus the crack.
Its simple enough to find it and weld it up...

Rich

Or you can use the Brad Mayeur outer kit over top of whatever rusted existing long you have left and tie in everything including the rear suspension. I hope to do this soon.

I couldn't bring myself to do that. I would remove the rusty metal, then weld in patch pieces. Then maybe a clamshell for extra strength, but only after knowing I wasn't covering up rusty metal.

I like the Engman kit, but if there are strength concerns past the firewall I would also run a tube from the rear shock tower tops to the inner longs, using a scab plate at both ends and knowing the rear one bridges a hidden gap between the top end of the frame and the shock tower.

I'm going to remove / "treat" the rusty metal first - but use the Brad M kit as my replacement solution.

anyone have the link to the Brad Mayeur long kit with some picures?

Well - here's a link to this very question from a couple years ago with some pictures (Bruce - you actually posted to this thread).

Outer Long Reinforcement

as you can see, the outer long overlay goes right up to the rear suspension console and spans that weak spot where the inner long is penetrated by the heater tube.

Rich

Oh yeah, I did chime in on the other thread. Looks like my early Parts Werks kits are the same as the Brad M pieces. They just came welded together. I remember the sales pitch was that in a car that was sagging, you could stick this under it and jack it up to straighten out the car. I was going to do that on an old project I had, but sold the car before getting that involved. So... I still have them.

I've got Brad's kits on my conversion car, and they do a very nice job of stiffening the car. They don't look original, but do the trick nicely.

I think any of these options will be a nice upgrade.

Rich,
do you have close up of that picture. I'd like to see the front part, did you have to cut the front fender and front door jamb?
Thanks,
Mike