[BRENT in 10-uh-C]

Too many questions to answer at one place so I am going answer in Red

I will be starting body work on a ‘29 sport coupe in the next few months, and plan to do the body work myself. I’ve had experience with installing patch panels on my last restoration, but rust is more extensive on this project.
Besides needing new rear quarter and wheel well panels, this sport coupe body is rusted through in several spots on the outer body curve just above the rear fenders and at least one rear subrail will need to be replaced.

I have not been able to find patch panels for the outer body above the rear fenders, or a partial rear subrail - does any vendor supply these ?
No one commercially manufactures these panels between the inner fenderwell and the rear quarter panel. You will need to form these either on a Pullmax or on a Tipping wheel, and you will need a shrinker at minimum. When we only need a small section to repair a sill, we just fabricate it on a brake. I am pretty sure that all you can purchase is a half-section for the sills however these only come in pairs (-right & left).

Assuming I may have to fabricate these parts, are there good sources of sheet metal suitable for these parts ?
What you are looking for is 19 gauge AK (or deep draw) cold-rolled steel. It is very hard to source. I get mine from Central Wire & Steel however I am pretty sure you will need to order multiple sheets.

I have an Eastwood stitch welder I bought 30 years ago - I notice Eastwood no longer sells these. Is there a better way to weld sheet metal these days ?
I am not familiar with the stitch welder but the preferred method these days is to fuse-weld with an oxy/acetylene torch, or followed by a TIG welder, followed by a MIG welder, or last & least by a stick welder.


I also see that SEM sells a urethane 3-part adhesive for sheet metal panels - is this a good substitute for welding metal patch panels ?
From my limited experience with this, Lord Fusor brand is the preferred brand in the collision industry however there is a huge learning curve to making a great structural bond. I attended a school on this and learned very quickly that it was not for us. Additionally, you will need to fabricate some specialty clamping fixtures for use during the bonding process. Based on what I know about this, I would probably recommend you steer clear of this unless you can find a mentor who is equipped to do so with the proper gun and fixturing.