I just did a 305 to 355/ 700r4 to t-5 swap. Never had a problem with cooling on my 305, but now if its cold I will run 160-180 on the freeway, if I hit traffic or around town it will climb to 230 or higher and never drop back down, even when I hit the freeway it only drops to 200-210....... I installed a 3300 cfm electric fan with a built in temp relay and it will fire up right at 180. I even installed a secondary 1300 cfm pusher fan on the opposite side and no difference.
My air dam is intact and I removed the fog lights and have no a/c so air is strait to the stock radiator. I bled all the air out, replaced the thermostat put a spring in my lower hose to keep it from collapsing (I saw this happening) and am running a Moroso water pump.... I'm hoping someone has had this obstacle and maybe fixed it with a larger radiator???? Its starting to warm up out here in Utah, just hoping to get my marrow in order before it gets to hot....... thanx fellers

 

Do you have a serpentine setup? If you do, make sure you have a reverse rotation water pump. Run a combustion leak test. There is a dye that will change color if there is combustion leaking into the cooling system. If this is the case, could be head gasket, head surface, or a crack in a head.

 

Bigger Rad. I had to lol

 

I did a 2.8 to 350 swap, I used a new V8 radiator that I bought from napa, and I have a flexalite black magic fan, mine would run hot in traffic, I bought a "high flow" thermostat from summit or jegs, and that did the trick. I think its a moroso brand, and it is 160*. Anyway, try a highflow thermostat.

 

Took me a long time to figure it out but I ended up drilling three small holes in my thermostat...motor stays right at 180 all day no matter what. I dont know if it allowed the thermostat to flow a little better or if I had an air pocket and the holes allowed the air to purge out when filling system or what but I do know it fixed the problem.

 

Step up to a 3 row aluminum radiator and you should be set.

 

thanx fellers, I knew it was time for a new radiator so I'll get one ordered tommorrow, I've got a 1/8 inch hole in my thermostat right now so we'll see how the radiator treats me, I'll update when I get it!