Let me take a guess.

Drivers Side?

It seems like there must have been a problem with the battery acid or something washing down the drivers side frame rail because I have rarely seen the E-body frame rails rust only at the T section. It's usually the whole rail.

Before you make a plan of attack I suggest you look over the entire frame rail even to the point of cutting holes on the floor to see inside the rail. I will bet that the rust is worse than you think and even the T-Caps won't be big enough.

If you find it is only the frame at the T section and choose to repair it consider this. The frame at that point supports the transmission helps to evenly load the body with weight and braking/acceleration stresses. But most importantly it is the anchor for the T-bars.

This a bad section to be trying to fab something up if you are only a OK fabricator. I'm not saying you can not do it but be sure you are confidant you can do it right.

Assuming you want strength and not going for a show car restoration, here is what I would do.

1. Check the entire frame from bumper to transmission cross member and decide how much HAS to go.

2. If it is localized I would clean the rust from the inside with a good blaster. Rustproof it then build a cap for it and weld it over the existing metal. Leave drain holes.

3. If it is cancerous but only on the trans cross member I would get a whole cross member from either a donor car or a new one. If it is cancerous in the rail I would get a new rail and do the whole thing right.

When you do start cutting it up I would get a bunch of RV load leveling jacks, or screw jacks and set the car on them before you start cutting.

Measure everything and then remove metal.

Position the new metal (maybe Tack in lightly) and measure everything again before you start the final welding.

Hope it helps