Our WP-20 has the same deal on the long back cap. 6" will solve your problem OR cut a 7" at 1.75" (or 1.5") and use that piece with the short cap like Rambozo mentioned. That's what I do.
On the .89, that is way out there, will take a large nozzle and a lot of gas. Start at .25" (8LPM) and see if that works for you, go a little longer if needed.