[folivier]

These and Lifeblue are both as close to direct replacement as available now, at least at this price range. Victron and a couple others are about twice the price.
The guys at Battleborn are very helpful with answering any questions. I have an older Prosine inverter which they say will work fine with these on either the agm or gel settings (don't recall offhand but BB can answer any questions). I also have a Victron Smartsolar controller which is easily programmable to the settings needed. Lots of good info on these at airforums.com
I'm replacing 3 8D gel cells, similar to 3 agms. BB recommended 3 of their 100AH batteries to be equivalent. So 6 should be a direct replacement for your 6 8D. Or you could do a test to see how many amp hours you use overnight and go from there. I also have a Victron BMV 712 so can tell how many amp hours I'm using over a time period. Or use a Kill a watt meter.
To fit these you could install 6 in the same footprint of 2 8Ds. So either remove the battery rack or use it for storage. You'll gain storage and save over 700#!
One other issue is charging via the engine alternator. These will take as many amps as possible to charge so it's possible to eat belts or overheat the alternator. Since I have solar panels I plan to install a battery shutoff switch between the isolator and the batteries. That way I can let the solar partially charge them, or generator/inverter, before I turn on the alternator. BB also has a battery isolation manager but I need to talk with them to see if this will protect the alternator.
This is a quote from BB about charging parameters: "Lets talk about charging in general before we dive into specifics.

• Acceptable range 14.2-14.6 volts on Bulk and Absorb
• Absorption time should be about ½ hour per 100 ah of battery capacity
• No Equalize needed, but if you can set it yourself, use 14.4-14.6 volts, this will help rebulk certain devices that need to get below 12.6 volts to rebulk. You can use the manual equalize if set properly to initiate a bulk charge
• Float 13.6 or lower is great, you can even run a solar charge controller that is not settable at a 13.8 volt setting. The confusion about not needing a float is a great point, thanks for bringing it up. We do not require the batteries, but since so many devices have this setting already, we ask you to set below the natural float level of the lifepo4 batteries. "

It appears that there is a lot more empirical knowledge about charging, floating, etc. than when I did this with individual Winston cells a few years ago. And in doing the research the last few months individual cells are actually more expensive when you add in the BMS and other items needed. I'd give BB a call or email them with specific questions for your inverter and other questions.
Just do it!