hi do you have to change the motor mounts when changing the motor from a 307 to a 350? thanks

I wouldn't think so. Both are for a SBC. I know there are subtle differences in some SB mounts but I don't see any problem.

from what i hear, both those blocks are identical, sept for the bore size

You'll want to read through this excellent article by David Pozzi for the where's and whyfores of mounts - but, yes, there are differences in the mounts - chose the correct ones and you will save yourself a lot of work down the road...
Link to mounts info - http://www.pozziracing.com/camaro_engine.htm

For 1969 Chevy used different motor mounts and corresponding frame brackets 327/307 and 302/350 engines.

The 307 motor mounts will bolt to the 350, the important thing is if you have 307 frame stands, use 307 motor mounts, if you have 302/350 stands, use 302/350 motor mounts, this will keep everything alined correctly.

They position the engine slightly different and could cause vibration issues if interchanged, if I remember right the 350 mounts will fit on 307 stands, (but not vise versa) and you'll get a high speed vibration, and it took me a long time to figure out what happened after an engine change :)

the main difference is the 302/350 mounts fit the engine slightly higher. They did this so the 8" balancer on the 302 / 350's would clear the stock sway bar.

Where the rubber motor mounts mate to the frame stands is wider on one type, to make it hard to use the wrong combination.

It seems most aftermarket sway bars will clear the big balancer with the short mounts tho. My Addco is dropped plenty low in the center and would not be a problem.

thanks