Lacking an NEC program, I had to learn the hard way what the wise gents above have shared...
Fortunately, I experimented with cheap aluminum electric fence wire, so no big $$ loss. (No worries guys, only 100 watts here... no wire fires.)
I would suggest a 'T' if you have a place to run the wires to horizontally... Otherwise 'guy' the vertical at the highest sturdy point (the tip top of some verticals would be too thin to support the downward pull) and run 3-4 wires down to insulators then the rest of the length use rope to anchor points. Also, you may want to guy half way up the vertical with JUST rope unless what you are using for a vertical is super sturdy, like pipe. (Only guyed at the top, it could wobble in the middle like a broken popsicle stick.) I have done exactly this with a refurbished/reworked Butternut vertical that is 25 feet tall. It is inductively loaded near the base with coils and capacitively loaded at the top with "umbrella wires", as they are called. (The wires are attached at around 17-18'.) It functions as a fairly good 160m & 80m vertical. If I were to loose the base loading coils and shortened the wires on top, I would have a 40m vertical just like you are looking for.
In contrast, I also have a 30m vertical with a top hat that is another 'rework a commercial antenna' design. I added 3' to the length of the vertical (at the base) and then added wire LOOPS at the top. The vertical is 20 feet tall and a 1/4 wave vertical for 30m needs to be 23 feet. So I added the loops to basically make up for the missing three feet... but there is much more than three feet of wire there. (And the shape and spacing of the loops is a bit touchy, by the way.) You would have to make loops considerably larger than mine and unless the vertical was extremely sturdy, it would be considerably top heavy. Not only top heavy but 'top loaded' in the wind...
A top hat acts like a bit of a 'sail' in other words. I would recommend trying umbrella wires or a 'T' in your case.
Pics and description of my antennas on my QRZ page, if you care to have a look: http://www.qrz.com/db/N4UFO
73 and thank you for your service,