Sparks and Speedo

Alright so hello everyone. I'm kinda hoping I'm posting this into the right section and it seems like it should be here. I recently bought a 1983 GPz 750 and I have been going through and replacing the basic stuff. So far I've done the following.
1. Oil/Filter
2. Air Pods (4 filters for the carbs)
3. light washing (it was dusty as heck).

I'm on where I want to replace plugs but I'm kinda stuck (I'll get to speedo after this).I've been doing some research and it would seem that most people recommend getting the NGK B8ES plugs. The reason I feel kinda stuck is because my uncle (person I bought from) put in stage 3 jets into it and retuned for those. Would the plugs still be the same or is there another one I should go for? I've always left bikes pretty much stock and don't know much about when they get modded so to speak.
Is there one that you would recommend getting?

Onto to the Speedo part. I noticed while going up the highway the other day that for me keeping up with other vehicles my bike was saying I was doing 90mph (So it seems almost 20 mph off from what it should be). The area I noticed this in was a 65mph zone and most people do between 65-75 on this stretch. Everything else seems to be working fine (exception being neutral light which I think is a loose wire) miles (trip meter as well) tick over as they should along with the tach showing proper rpm and volts when the button is pressed. I have seen that a lot of the time the cable goes but it seems to me that they are referencing the milage not working properly when talking about this.
Any thoughts?

I am working on hunting down a manual and then I'm going to go from there for everything else. I just wanted to post here to get some ideas on where to head from people who have owned them for awhile and have first hand knowledge.

The Kawasaki service manual lists B9ES spark plugs for the ZX750-A1. there may be a sticker under the seat or behind a side cover with tuneup info. I don't know what would be appropriate with pod filters, but you should keep a close eye on the spark plugs as the pod filters flow more air, so the bike could be running lean. The speedometer may be defective, but I would check the cable just in case. it's pretty easy.

Manual. 750turbo.com --- Index .
1980-84 KZ750-4. 1983-85 ZX750. pdf manual. good scans.

Wiring diagrams. mbsween. bike-night. KZ750L3 . Scroll down & click "The Bible" link.
Note: some of the wiring diagrams are mislabeled in the links: the 1981 Ltd is an H2, the 82 is an H3, & the 83 is an H4.