Plans for RCF dual 18"

I'm currently looking at the idea of getting a professional cabinet maker to build me two G Subs loaded with RCF LF18X451 drivers but someone has said the ports might not be big enough because the LF18X451 drivers move a hell of a lot of air. I don't know the internal volume of the G Subs (taking off for braces etc..) though so I can't calculate how long or big the ports in the G Subs would need to be.

Like i've said I mean 3db.

Gsub with all that baffle inside aint big enough. I'd start with the below and get used to sims. Having said that if you leave those triangle ports un-glued you should be able to play with various lengths by ear and choose what suits?
http://www.fane-international.com/downloads/Fane%20EN21802%20Enclosure.pdf

I put it into WinISD and tried it with two RCF LF18X451 drivers in a G Sub, I adjusted the port width to 102mm instead of the 120mm width on the original G Sub plans and aport length of 221mm instead of the 193mm of the original G Sub plans. This below is the frequency response it gave me, box tuning 35hz and is -3db down at 38.5hz.

Look at your port velocity.

Those results aren't great in terms of SPL or cutoff frequency to be honest.