Red tops are names used for a series of batteries with different capabilities and different design. People use the name interchangeably for all. For instance, there is a 6vdc optima red top and a 12vdc optima red top. They run from 600 cold cranking amps to 800 and 1000 cranking amps normal.
Most people don't realize, but Optima actuallyrecommends yellow top, which are normally considered deep cycle in the public eye, for commercial application or large diesel trucks with lots of electronic toys. Even in that case, yellow top are not yellow top as there are many differrent types of yellow top with different salient characteristics for different applications.
I recently purchased 4 of the commercial grade yellow tops with 750 cold cranking amps for use in my golf cart, replacing 6 troy high dollar batteries that gave me the capability to play a full 18 holes if I needed. The yellow top are not as big and the dealer told me I would get an hour and a half useage in lieu of 3-4 hours like with the 6 troy batteries. I only use it in the yard so an hour an a half is fine, as long as it produces enough torque to get me up the hill from the lake. They have been installed and it does. It will spin the wheels on the golf cart, and start up a deuce fully equipped with electronics, so I now have dual capability with my golf cart batteries where the new $1200 set of Troy batteries could not be used for anything else at 8vdc each.
I will always have a slave set of batteries charged and ready to go. I will always have a set of generator batteries. Check the CCA of your red tops, and if they are 750CCA or better, they will definitely do the job and they will charge much faster than lead acid. The 6TLs are usually overkill, but have lasting capabililty. I would much rather go with suitable red top, yellow top, or even Wal Marts best high CCA batteries then buy the 6TLs.
You will have to modify the battery rack to stabilize them for rough terrain, but it is well worth the cost savings, and the fact you have a dual purpose battery you can put in your car, tractor, generator, or other civilian application and crank if you need it.