What gain will I see in my time slips

Rough estimation (VERY rough) is two hp per cfm is possible, if everything is right. Therefore, if your porting goes from 240cfm to 270, you can theoretically pick up 60 hp. How much of that potential 60hp shows up on your timeslip is ENTIRELY dependant on how good your car is.
You can go to wallace calculators, and investigate how much power you have now, and get an educated guess as to what it might do with more.
Edit: 60 hp from just porting would be a huge increase, i wouldn't expect to see that on my junk. Still, pocket ported 906 big block heads MIGHT flow 240, edelbrock claims(repeat, claims) 290 from their big block rpms.