Not exactly sure which area you are considering adding rod holders.
I purchased a set of factory hardtop rod holders for my Marlin last year. It came with six welded rod holders and gets mounted to the hardtop itself. It was a simple install, took about two hours because I took my time and wanted to avoid costly mistakes. Most Gradys I see with hardtops have this type. Took about ten days to get them to the dealer if I remember correctly. The rod holders match the four hardtop leg mounted rod holders perfectly. Cost wasn't too bad either, around $650 plus shipping.
If you meant you want to add rod holderrs to the hardtop legs I have used the Tacos on my last boat before I sold it. They were on the boat for about four years and did not move once. They held up to the enviroment quite well, I would not hesitate to use them again. I prefer the welded on type, but in reality they are not practical to have installed on a boat with wires run through the hardtop tubes, curtains (and tracks for same) installed, many flamable items nearby, etc. I may add a pair of clamp on myself in the not too distant future, not sure if I will go with Lee, Taco or another brand.
Not to highjack your thread, but does anybody know what brand the hardtop rod holders are on a 2006 Marlin (probably the same on all Gradys of this era)? I would like to match what is already there with clamp ons instead of welded.