that is a whole lot of cam for a 305, i would say that would be a healthy street/strip cam in a 350. what heads are on the 305 if they aren't th 4416 HO 305 heads then they are pretty much garbage and need to be scrapped. i know that you should already know this but the 305 is certainly not the engine you want to start out with as a street/strip motor, they are a efficent low end light weight tow motor, they make next to no power above 4000-4500rpm. pretty much all the non 4.000"+ bore sbc's are not a good motor to start with when wanting to build a performance engine.
about the largest valve's you could go with would be 1.94 intake and a 1.5 exhaust as you will still have to have about a combined .060" clearnace between the two valves and the valves and the cylinder walls. so you would have to see what all is in your friends heads
how about this you find out what are all the specs on your friends TF heads and let us know and we'll tell you if it's a good idea or not.