Wow MJD I just did almost the same project putting together a whiplash that had been stored away. I think these guys are selling JETT products. They told me I had to buy one of their pipes and it was not better then stock OS and was not even comparable to mousse can.
Below are some links of my whiplash, but now has been modified with correct prop tru turn adapter and aerospace APC carbon fiber at engine mount and c/f at leading edge.
These deltas need exact CG, that means if you run a big tank, you need to make sure with the add (difference in weight) you need to stay within the CG range. I had to run 12 oz for 4 mins. I am at 2" with the add 5 ozs difference add to tank area for balancing and 2.5 empty w/o added weight. MY 3.3 hp 2 long stroke YS 91 SR with mousse can has a weight of 19 oz. To make CG I placed MG digital servos at rear tail end and centered on alierons. If you are interested I will link another picture. But it reduces flutter to have servo centered. Also used 1000 mah a123 6 volt lipo, 1500 standard lipo and regulator in tail section below fins. No weight added just moved regulator around.
I am not sure about the OPS engine you mentioned but I think big bore is the only way to go and not to worry about a small difference in engine weight. A high speed small prop is way too hard to hand launch. i am up to 20+ flights on my whiplash. When I ran the 10 10 prop, it was fast but if hand launched at steep angle (pitched to far up) it would nearly be uncontrolable till it got up to speed. I ve seen other guys encounter the same thing with hand launching deltas. I felt safe with 12 12N prop but only got 13k rpms. I am going to run a thicker hub BOLLY carbon fiber 11.25 by 8 this wkend which should hit higher rpms and be easy to launch, which works well with my tru turn prop adapter as the 91 sr was designed for helicopter.
WE flew the shark coroplast pylon combat planes and they got top speed and launched best with a combo of a 25 fx mousse and just a 8 by 5 apc prop. For hand launching you want a bit larger prop and less pitch but with high rpms you can stil get the speed.
http://www.rcuniverse.com/gallery/ga...berID%3D235242
http://www.rcuniverse.com/gallery/ga...berID%3D235242
Let me know if you want me to link and/or post if someone will tell me how from the top view. Please let me know how your project turns out. With C/f reinforcement I think this delta will last for a long time and boy is fast. Good luch