Step 5: Replace the blade with a fixed string head
(more photos coming)
Note, with 3 to 5hp, you can run as thick a gauge of line as you care to. The minimum needed is .130! ! ! I recommend .155 or .170, or if you can spend it, that "flexiblade" is unbelievable for cutting power and almost indestructible. I've used the flexiblade string and it's wicked, but it's expensive and only works in quick-change type heads with ratchets.
There are several ways to do this. You can buy a commercial fixed "4" string head for brushcutter mowers (about $30), that fits the blade fixing bolt you have. You may have to remove the blade-to-crankshaft adapter and attach it to the bare shaft (not recommended, but it works well, just has less strength than other options). You may need a longer bolt. You may need to find or fit a washer to adapt the string head to mower shaft. Most likely the kits with multiple washers will have yours.
If your mower has a blade-to-crankshaft adapter with a large flat "washer" on the bottom, this works great to just bolt a commercial fixed-head to it.
You may also find some quick-detach fixed string heads (2 strings) that bolt up easily to the bare shaft and protect it (they will place the string path higher in the deck, note). These are great, as they allow you to use less string, and make string changes a breeze.
You can also MAKE a four string head from a worn out blade (labor intensive).
You will need to remove the blade, clamp it in a vise, and saw off the ends/wings (keeping 5" of the center piece), grind smooth and round the edges, and drill in the pattern shown or similar, to attach the strings. Chamfer the holes to prevent cutting the strings with centrifugal forces.
You could also MAKE a four or eight string head (leave short strings on and add new ones to opposite sides) for maximum cutting potential. Draw a 5" circle on a piece of 1/4" plate steel, cut out the circle, drill central hole, check object for balance as a flywheel, and drill pattern for strings.
Do not make it out of wood. I tried that, and it worked, but the holes wore out and eventually it broke. I made one before with a commercial aluminum fixed-string head and it worked well.
examples, with many similar products available.
Oregon brand string head $30 with adapter washers and nuts http://www.amazon.com/Oregon-55-130-Trimmer-Head-Fixed/dp/B0018U2CWU
Echo Rapid Loader head from Home Depot $30 http://www.homedepot.com/p/ECHO-Rapid-Loader-Trimmer-Head-21560059/100056334
Oregon Jet Fit Flexiblade compatible 4 string (the ultimate) $60 http://www.lawnmowerpartsoutlet.com/Oregon-Aluminum-Jet-Fit-Trimmer-Head-4-Line-55-404_p_4677.html
WARNING: Obviously, you will need to kill the mower to change strings.
Note: You cannot use this modified mower to trim lawns as a trimmer for cutting up to fences and bricks. The wheel gets in the way. You need a conventional string trimmer (portable or wheeled) for that. This is for beating down hiking/biking trails and overgrown lawns.