I was wondering if a set of 18" wheels with the following offset fits on 997.1 GT3 without any fitment issues.

Front: 18*8.5 ET 44
Rear: 18*10 ET 50

Additionally, what is the best 18" track wheel offest without spacers?
Thank you for your help in advance.

Stock 997.1 GT3 is 8.5" w/53mm offset front, and 12" w/63mm offset rear (net, i.e., including the 5mm spacer). As you can see, the cited specs would not work well.

Wheel diameter has little/nothing to do with offset; wheel width does. Most wheel manufacturers I spoke with use 63mm rear offset for a 12" rear wheel. That said, I still use a 5mm spacer, so 58mm will work fine without spacer.

NO!!!! wrong width and offset

for 18" you need:
8.5x18 et53
or
9x18 et50

12x18 et63

Correct- the 9" work great up front. I just ordered that config from CCW and am very satisfied.

thanks for your help guys. it was very helpful