Fuel Sending Units for 84 3400 Express?

[ExplorGM]

Greetings!
Our boat has relatively new gauges but both fuel gauges are reading low. We filled up the tanks yesterday and both gauges are showing much less than a full tank. Before filling the starboard gauge showed 1/2 full but only took 26 gallons for example. Thinking there is either an wiring problem or the sending units are dying...
Although we have receipts for "professional" work done on the boat for the last 30 years, I've found some funky wiring already so wondering if the guys that did the guages might have done some mickey mouse wiring... Anybody know exactly where the sending units are? Hoping not to have to tear up carpet to access a hidden hatch... I read on some of the other posts for other boats on this forum that there may be a hatch somewhere... Would new sending units even be available for this boat?

Speaking of fuel... can anyone recommend a good "bulk" octane booster and a lead additive? The specs call for 88 and our local dock only has 87 unleaded. Not a huge difference but want to take care of these engines as they have only 30ish hours on the rebuilds.
I met with the Manager of the lake's shop that did the rebuild back in 2013 but he did not know if they used hardened valve seats (the parts came from another source) so thinking I might need to keep up on the lead just in case? I've used octane boost and lead additive for my 69 Buick with only a 20 gallon tank, but with 270 gallons to treat I am looking for the least expensive solution here.

Thank you!

[CptBill]

Moeller reed switch, try there web site, it solved my sending unit issue

[ExplorGM]

Thank you! Now I need to find the darn things!

[ExplorGM]

Also, I was able to communciate with the person that owned it from 1987 to 2018 and he said the gas gauges have always been off! I'll worry a little less but still pursue an R&R once I can figure out how to get them out.