360 running hot

I rebuilt a 90 360, new rods, KB107 pistons and old crank. The crank journals showed no wear. It has fine oil pressure, runs great and will idle for hours and not overheat. It has about 200 miles on it plus more then a few hours idling. The heads are new magnums, the block was well cleaned at the machine shop. The cam is a small roller, 209 at @.050 and .459 lift. The timing is 16* at idle with the vacuum to the distributor disconnected and 32* at 3,000. It runs strong but on a long drive today it got hot. The radiator is out of a 73 Valiant 318 with air and it has the 8 ? blade water pump. I'm using the shroud from the Valiant. I'm hoping it's not too small of a radiator, maybe less timing? It seems to heat up while driving around 3,000 rpm. With the radiator cap off I can see good water circulation.

What do you call hot?

It didn't boil over but hit the H on the gauge. Probably 210 to 215. Never did that before.

My 74 Dart came with a 3 row 26" radiator.What you got in it 2 row 22" ?

210-215 degrees isn't bad, worry about it if you get up 230 or higher.

I have been running that same rad since 1999. No problems @ about 420hp.
An I.R. gun will be your best friend.
Overheating at speed is usually a problem with heat transfer through the rad,especially if it has a condenser in front of it. The condenser fins could be the problem. With no A/C, It could be restricted core-tubes or separated fins. At 3000rpm the water should be circulating pretty good.
But cruising at 3000, the engine will want well over 40* timing. That's where the Vacuum advance comes in.Without the Vcan, the engine will want to run a little richer.If you don't start the fire early enough, the pressure peak could occur very late in the power cycle and it is possible for the still-burning charge to continue burning in the header/manifold.
That cam at 209* is pretty small (about the same as a teener cam), so the power-timing will need to be carefully monitored to keep the engine out of detonation. If it was mine, I'd probably run a little less initial timing, a little more power-timing,and perhaps bring it in a little later. And then I would install the biggest Vcan I could find. Ima thinking I might try 12@1000 and 34@3600. This works out to .85* per 100rpm. So at 3000 you would be running {(34-12)x .85}+12= 31 and I would try a 20* can on that, to total 31+23 =54* and that should be close. The rest is in the tune.
If you are running open chamber iron heads,she might want a little less Vcan or adjust it to drop out a little faster.The Vcan may be limited or swapped out for a smaller one as needed.
But I just gotta ask;why are you cruising at 3000 with such a small cam? That is a gas-miser cam, and has the potential to pull over 30mpg with an overdrive.And make plenty of torque with a starter gear of around 8to9. That cam would be great with an A500 and 3.55s. The starter would be 9.73, and od would be 2.45.
75mph would be 2200 to 2400, depending on tire diameter.I was able to tune a 600Holley on my 367 to get 32mpgUS;with 10.8Scr (aluminum heads),and with a 223* cam, and an A833/ 1.97 final-drive ratio. I think your cam could do better.

I was at 3k because that motor runs so sweet there. I'll drop the initial to 12 and see what happens. The car feels really good at around 70+> The gears are 3:23s and it has a 727 right now. 4spd going in this winter I hope. The radiator is a 22" 2 core, a little small I know. I had the original cam reground at Bullet Racing cams. I gave him all the specs and told him it was a cruiser. It does have some top end. I've been ironing out the bugs and this cropped up. I'm hoping I can get by with this radiator. I'll try this tomorrow and see what happens. I do have a IR gun and have checked it a few times. It never ran this hot before.

OOPs, the teener A/C rad is supposed to be a 26. Or at least Mine was/is a 26.
But your combo should be fine with a 22.

Dropping the initial to 12* with no other changes is the wrong thing to do. That engine wants more cruise timing; a lot more. Pulling 4* out will just make it worse.

Keep your eye on the gas-gauge when the temp gauge goes up. Those two run off the dash-gauge regulator. If the regulator sticks, it will send both needles to the top.

Just a little update. I'm rebuilding the dish washer, then back to the car. The door switch/lock is a plastic piece of crap. I ordered a new replacement. Back to the car,I have the MP performance dist. I'll set the timing on the mechanical to come in at 34*/ 3600. and back off the initial to 12* at idle. This motor has torque from the git go!!! Two questions, on the adjustable vacuum advance. Which way do I turn it to raise the rpm and what does the 8.5 stamped on the arm mean??