Depends on what you are trying to accomplish with the UV.
3500 LPH is the same as 900 gph, a strong and rapid flow rate, not slow enough to be or allow an effective detention time,
the detention time (flow passing past the UV bulb) needs to be long enough to scramble the organelles of any pathogens you want eliminated.
If it were me, I would Tee off that return line.
With the Teed off section of plumbing choked off enough , to permit a much slower rate and longer detention time, such as a flow in the single digit rate .
As you see in the chart posted above , flow rates effective against parasites are only in the single or double digits, depending on what you are trying to kill.
With a flow rate of even 100 lph most pathogens would spend so little time in front of the bulb, that they would not be in the least affected.
Above is an example of a controlled flow line (upper right hand corner of the shot )that is Teed off from the rapid flow line.
The flow from the left part of the manifold is a strong rapid flow (5000 lph), on the left most line the "red valve" is fully open
on the right (and in the shot below) that red vale on the Teed off section is partially closed (choked off) allowing for a much slower flow to be sent thru that separate line, back to the sump. This is the type line a UV unit should be installed on.
If any of the above isn't clear, just ask about what is mot.
As an example, an Ick parasite, or newly hatched Lernaea needs to spend a very long time in front of the UV bulb (slow rate (5 lph) )to be even remotely have its organelles scrambled by UV..
An adult Lernaea (below) might need a half hours detention time, in front of a bulb, but already attached to the fish, UV would be useless.
A UV light is "not a panacea" that wipes out all diseases,
unless that phage (infector), is planktonic, and passes in front of the UV bulb "long enough", it won't be eliminated,
There are plenty on non-planktonic diseases, that are external (living on the skin of fishes body) passed by fish to fish contact, that won't be eliminated by UV.
There are plenty of internal diseases passed when a fish eats prey item (such as a snail, or other smaller fish), that are unaffected by UV..