You don't want 225/40-18 (I think it's a typo):
40% x 225 = 90mm = 3.5" sidewall --> 25.1" OD (stock is 25.7")
There's a significant difference between 245/40-18 and 245/45-18:
40% x 245 = 98mm = 3.9" sidewall --> 25.7" OD
45% x 245 = 110.3mm = 4.3" sidewall --> 26.7" OD
When you compare these to your stock size of 225/55-16:
55% x 225 = 4.9" sidewall --> 25.7" OD
you can see why 245/40-18 is highly recommended...
But you want larger profile, so I would suggest going wider with the tire, which would increase the OD in smaller increments than just bumping aspect ratio (A/R). You saw above how going from 40 to 45 A/R increased the OD by a full inch!! That's too much and your tires would probably rub. On the other hand, going from a 245 to 255 wide with the same 40 A/R increases the OD to only 26", and a 265 section width yields an OD of 26.3"... So those sizes (255/40-18 or 265/40-18) might be better options for you.
My $.02: I don't suggest messing with the OD of your tires. You are screwing up your suspension and you have to worry about tire-rubbing. I highly recommend staying with the stock OD and using tires that make that happen. This is the sacrifice that goes with large rims -- low-profile tires and worse ride quality. But you gain leaps and bounds in the appearance department, IMO.
I have had 18" rims (with 25.7" OD fitted tires) on my 98 for about a year -- and I love them and don't care that the ride is a little harder. I'm just more careful of potholes.
FLASH = 98 V6, chrome yellow, black & blue accents, 146k daily driver POWER
: divorced exhaust with glasspacks; BBK CAI, TB, UDPs; (shift kit in the garage) STYLE
: black headlights, black taillights, black grille, black hood pins, black window tint, blue hood & side scoops, blue billet fuel door, interior billet, 12000k HIDs RIMS
: XXR 006 black spoke with blue lip; 18x8.5 front, 18x9.5 rear TIRES
: BFG KDW/2 245/40R18 front, 275/35R18 rear