The paint on my stock front left and right speaker panel is pretty worn. Is it worth painting it in black matte spray and installing them again?

Just so you have correct info, Nissan did not offer speakers in this location - what you have were added by a previous owner.  With that said, the panels the speakers are mounted to are generally referred to as "kick panels".  A matte black repaint would be pretty accurate.

I bought the msa kit but the speakers magnet are too big.

Derek I believe is on this site as well as on hybridz. He makes these for the 280z as well as for the 240z. I have a set for the 240z and they are very nicely done. 

https://www.ebay.com/i/253187800430 

The 240z front speaker kick panels are also available from MSA. They worked great in my 71 240z.

http://www.thezstore.com/page/TZS/PROD/classic08c02/50-5358

 

Edited October 19, 20177 yr by w3wilkes

i bought that kit but the 5.25 speaker magnet were too deep and thick and they did not fit flush. Maybe i should keep shopping around for speakers of thinner magnet.

Interesting... The RetroSound speakers in the MSA speaker kick panels had no fitment issues at all in my early 71 car. 

Edited October 19, 20177 yr by w3wilkes

I bought normal jbl 5.25 speakers. they dont fit!!

Edited October 19, 20177 yr by jalexquijano

Are you talking depth hitting the back or they won't fit in the hole?

Magnet too big hitting hole

You can't trim it? As long as the magnet is not bigger than speaker it shouldn't be an issue. Didn't the jbl speakers come with a template? Measure hole width vs template. 

Magnet hits the chasis