Rob,
Firstly you want to take that 09154 24" palmyra broom head back to HF and swap it for a 90154 trailer....
I'd say make your tongue out of 2"x2"x1/8" square tube. It needs to be 48" from the centre of the ball to the front of the cut-down HF trailer frame and an A-frame tongue in this tube would be OK up to a trailer weight of 1300lb according to my
tongue strength page, based on the fairly strict Australian trailer rules.
But if you want to build a Widget on a HF trailer, it would make sense to alter the trailer profile so that it suits the HF trailer. The red lines shown below are the two changes needed:
Then the only change you'd need to make to the HF frame (apart from the new tongue) is to move the second cross-member, if you want to include the dropped floor in the Widget plans.
If you wanted to do this, I'd draw you a new profile with these changes.
Andrew