i was wondering if it is alright to put 19's on the front and 20's on the rear? would it effect on how it ride?

why not, corvette does it with 17's and 18's

would it need special modifacation to do this

Here's a POST from when I asked a similar question.

The post is kind of rambling but the point of going with 19's in front and 20's in back is that the amount of rubber between the wheel and the road "is the same" for both the front and rear tires - approx. 3.5" of sidewall. If you put 20's in front you will lose more of the sidewall.

Plus, I would think that the VDC and your Speedo would love this combo since it's the closest to the stock wheel diameters. Maybe one of the track experts can comment but I would think it would be better for cornering too(?).

04, TB/W, Nav, Aero, 6MT, Z-tube/ZWT, etc...

Sure you can do it, but you need to have the same rotating diameter via tire size. As far as handling goes, it all has to do with the size (width/offset of the wheels) and the weight. I would not put and overall smaller rotating diameter on the front vs the rear. There are drawbacks to this.

'03.5 Silver AT Sedan, Premium, Aero Kit/Package, Sport, Winter, Stillen Air Box, Z pipe, Polk Tweeters, Clifford RSX 3.5, Gram Light 57F Pro's (on the way), Hotchkis sways, Borla

as long as you choose the size of tire that's closest to the stock diameter/circumference, like: 245/35/19 front and 275 or 285/30/20, it should work.. good luck!