will the stock radiator from a 305 cool my 355 when i swap it in?

It should as long as it is in good shape etc.

I am having problems with mine though because the fan is stock, and the gauge is lying to me. I am going to make a hokey cardboard shroud to see if it will reduce the temps in the radiator. If it does, then I will make a nice one out of aluminium or stainless using the cardboard one for a template.

I swappped my 305 for a 357, 11:1 compression, with iron heads. the stock radiator, with single electric fan (factory), and a 160 stat, keeps it plenty cool idling, and for the limited times I have run it down the road. this is a drag car, so it doesn't get to idle in traffic. when I run it hard the temp rises pretty quick, but it hasn't gotten over 185. I also haven't run it in really hot weather. so what I am saying is that my setup is borderline, it will be fine for the track, but not quite up to task on really hot days in traffic.

Well mine is only a 350 (no boring done), but my stock radiator (with 180 thermo, and fan mod), the car doesn't get over 180 in traffic with the A/C on. How does it cool the car now?

its cooling pretty good... i think i gotta take out my rad and give it a good wash ... its prob fulll of ****.... but why does the a/c cause my car to rise real high to about 210?

High condenser temps will raise the temp of the air before it reaches the radiator.

On another note, my temp gauge is definitly lying to me. Yesterday it was pegged on 260* in the red as the car was at a high idle, but an RTD (very acurate + or - .05%) that I installed on the water passage before the water neck on the intake stayed at 195* untill I brought the idle down, and even then it only shot up to 200*. Just somthing to keep in mind.

I also read somewhere that if your A/C system is not fully charged, that it can cause the water temp to raise. Not too sure if this is the case, but your compressor is supposed to cycle on and off is it is fully charged. If it never cycles off, then chances are you need the A/C charged up some.
*NOTE* If this is the case, then you will want to fix the leak also, otherwise it will just leak back out again. Freon does not evaporate/burn up in the system, so if it is low, there is a leak.