Dear Smart and bob,
There is nothing wrong with a dual purpose utility/tear, even one overhanging the frame.
Other than the bolt down factor, for which there are many easy solutions (see r/c's link above), there are some issues of frame placement, axle placement, and tongue length you are going to have to spend some time noodlin' over.
Typically a tear has more rear weight than front weight so the axle has to be placed further to the rear. A typical utility trailer has the weight centered so the axle has to be further front. Your plan to extend over the rear 18" would put about 2/3 of the total loaded weight on the rear member of the HF frame which may not be sufficient support.
A Teardrop axle placement will give you the tongue weight needed for towing a tear but then make your utility trailer tongue weight very heavy.
Overhanging the front of a HF trailer may not give you sufficient tongue length for either travelling stability or turning radius.
Remember tongue length and axle placement are the two most important SAFETY considerations in the building of any teardrop or utility trailer and they should not be compromised. So, that said, be careful, think hard and long.
Remember you have never wasted time reading this board. The only waste of time is thinking you know "exactly" what you are going to build before you start reading this board.
