I'm 6' with long limbs as well, so leverage has always worked against me....
First, I recommend reading this T-Nation article, "Why Lurch
Won't Grow (Tall Man Training)", by TC:
This is going way back, but after about 3 years of bb I hit a wall with my arm development, and was hovering around 18.5". And even though I had big Tris, my Bis always outpaced them by a bit, so they were my weak link. Long story short, I started doing weighted dips (bw+90lbs.), and within several months I broke the 19" barrier.
Now, 20 years later, after two shoulder dislocations, I feel uncomfortable doing dips. I've only just started rehabbing my shoulder with stretching and PT exercises using light weights for my rotators, etc. So, anyway, once again, my Tris are lagging a bit, in comparison with my Bis, so I came up with an exercise that gives me the same feeling and development potential as weighted Dips, but without the shoulder strain:
Using an overhead Cable, like the one you do Pulldowns with, attach about an 18" chain to the top coupling with a bar (preferably a cambered bar) attached to the other end, so that the bar itself hangs about halfway between your chest and your navel.
Then turn around, facing away from the bar, and take a shoulder-width grip on it, behind your back, and keep your elbows in tight, so that they are facing backwards. Basically, you should be able to stand on tiptoe to get your grip and start the movement. But once the triceps are under tension, you will be standing flatfooted. If this is not the case, adjust the chain length accordingly.
It's just a Tricep Pushdown, but done behind the back. It will help if you lean forward a little while performing the reps, pushing the weight pretty much down from the middle of your back to your butt.
It sounds like Bench Dip, but it is far less stressful to the shoulder joint, because the Bench Dip requires you to move on a forward angle, away from the bench. But if you have mounted the bar correctly with the Reverse Pushdown, you will be pushing the weight straight up and down, behind your back, using only your tripeps, if you keep your elbows pointing back.
Anyway, I haven't been able to find any other movement to duplicate the Tricep stimulation I get from these, and since it is a cable movement, if you aren't very strong at first, you can just use light weight and work your way up. Maybe, give them a try, and see what you think.....