HF trailer & puffin like top? Axle where/

Hi people, after a year of lurking, finally starting my TTT. Have a harbor freight 90154 trailer. Plan on cutting one spar, to fill in the folding side seams. (half on each side). Then use the other spar (metal frame at fold area) to extend the rear about 18-20 inches. Trying to keep the tongue weight 100-130 lb. for a 1 1/4 receiver size, using a toyota matrix to tow. Read about moving the axle back to the 35" mark, but with the added end, it comes to about 43 inch. Do not know the original measurement yet, have not pulled it out of the boxes yet. Build it like the puffin, but with w / ac possible on the back wall. 5'w x 10 'L and 5 ' high, are my present ideas. Trying to be like the puffin, but with a possible high back opening trunk? (storage for traveling weight distribution on the back end) Open to input and opinions.