Lets Roll, please don't take this the wrong way, but what you have there is going to hurt someone.
You have a couple of serious no-no's in structural design going on here all at the same time. Especially in something designed to support a human.
Multiple load paths are desired for redundancy and for stiffness. While your truss design supporting the long portion of the boom is a good idea, it is not really large enough to do a lot of good. A truss is designed to avoid flex. The tractor end of your top piece are welded to the edge of two pieces of what looks like 4" angle. Under load that angle will flex forward and deform, this will stress your welds, particularly the forward one. The forward attachment point for the top piece is only attached with a weld along the lower skin. This component in this application is in a nearly total tension situation and you are really only making use of the lower skin of that tube. You could get the same strength for 1/3 the weight by just using a piece of flat strap.
In a structure such as this you want to avoid points where force is concentrated. The point where the basket fork is attached to the end of the boom square tube is one of those places. Because the boom does not provide any other options for multiple load paths at this point, I don't see any way to easilly fix this short of building another truss(more weight). The point where forks meet the backplate/forks bend is another such point.
Not even counting the weight of the steel involved, a load of 250 pounds(man+tools) at a moment arm of 15' equals a downward rotational force or torque of 3750 FT/LB. All this force is concentrated at the point where the forks are bent. Remember, that is not including all the weight of the steel. I would say as an estimate there is at least 300LB of steel involved. If it's CG is near the middle of the structure(8'?) then there is another 2400 FT/LB of torque added to the point where the fork bends. Add any form of moderate acceleration(2 "G"?) to the load at the end from a small bump or jerky hydraulic movement. That's over 12,000 Ft/LB of torque on the non triangulated point where the forks are bent. What was the load rating of those forks again? What is the curl rating of your FEL hydraulics? I think that much load applied as a shock would most likley rupture your curl cylinders or blow a hydraulic hose. The real load point if the fork/backplate and curl hydraulics survive is where the FEL arms are attached to the tractor. Again tremendous loads at that point. What is the breakout force of your FEL. In fact if you applied what I would estimate at around 14000 LB in a 2 "G" event to the top of the FEL lift cylinders, those cylinders would most likley collapse/buckle/rupture.
The flex present in the design would turn every upset into a series of high load oscilations.
In a ideal perfect static situation that thing may lift 250 LB in the bucket, but I would guess it will be exceeding every design load rating on the FEL to do it. Unfortunatly, the reality is that life is not an ideal perfect static situation.
You have to ask yourself, what are the consequences of failure? Are they worth the potential gain from success(trimming trees)? Please don't put anything you care about in that bucket.
Absolutley positively gotta get up to trim those trees? go buy a surplus boom truck. Telco and power companies sell off used equipment all the time.
