What you need to do is raise bike on lift and remove the rear shocks then measure the eye to eye distance of the mounting points to see if there is enough room to fit them with out the front of the swingarm hitting against the fuel tank. That is what will limit the length of shock you can install.
I did some geometry studies years ago, with the same interest in mind and discovered that a 1/2" longer rear shock would actually raise the rear end nearly 1".
Of course after I installed the aftermarket 5 gallon tank in my '04 the swingarm no longer had room to allow enough movement so I wasn't able to fit them.
The cross member of the swingarm in front of the tire moves up and down in a molded notch in the rear side of the plastic tank. It only has so much room to swing in.
Check it out before you spend good money on something that won't work.
One thing you can do is slide the front forks down in the trees a 1/4". Not much I know but it is at the front of the bike where the footpegs are which are generally one of the first things to touch down.
Thanks for letting me know about the fuel tank, I wasn't even looking there. I was just checking the clearance at the exhaust and wondering about the belt tension. Both should be fine with a half inch longer shock but I'll need to check the tank.
These 15-mm extensions give you more ground clearance at the footpegs, which is a big advantage in faster turns.
Made from stainless steel, they simply screw into the fork tube top and are clamped with the upper triple tree.
The extension fits into all standard forks of the V-Rod models up to the construction year 2011, except for VRSCF and VRCSR.
I got 2004 with 5.3 gallon tank with 14 inch shocks on mine and i put 21 inch rim up front and raised my whole bike 1 inch higher then stock Rides the same but i can lean alot futher now with out scraping the foot pegs. It helped alot