In addition to the good suggestions above:

Fasteners are free, some folks use other materials that are lighter. Of course, optional things like radiators needn't be held in with 6 stock bolts, two small tabs and two small bolts will do. That's just and example.

Suspension rules are pretty open. does the car run struts? those are pretty open, and significant weight can be saved there, Look at the inserts too. Swaybars. Big savings possible here. Hollow bars, with alum arms.

Stock systems that are optional. Everything goes. (although I don't buy into the 'remove the heater core because it's attached to some housing that the AC air ran through' mentallity). Anything that is open for removel, remove. Anything that is optional, (mirrors) consider weight when replacing.

Tires. They don't all weigh the same, and their weight 'costs' more as it's rotating and unsprung.

Exhaust. Stainless and or titanium. Significant potential for loss, and it can last longer.

On the Nascar door bars, read the rule carefully. Your right side should be the bare minimum to meet the exact words of the rule. (example: When they say "project into the door opening" they don't say by how much...you don't need the crush space, so 1/8" projection with ONE bar is enough to satisfy the rule requirement, IIRC. )

Seat. Buy smart and save 7-pounds or so.

Clutch and PP ...there are some savings available there, and again, thats rotating weight.

Guages. Remove all stock ones, replace with small dash panel and data aq system. Saves a bit of weight, but gives you a record of things you might be missing on track. (An oil pressure fluctuation in left handers that could rear it's head later, or a voltage fluctuation that explains that weird miss) AND you can review your driving which always makes you faster._

Sorry about this one, but eat more fruits, salads and exercise regularly. Or more. Helps during a three hour stint too.

Finally, read the rulebook, start to finish and think about weight in regards to every line.