Boil Over Help Needed

Hmm...sorta stumped and looking for ideas...

have an 84 camaro with a near stock 305 4bbl. it boils over teh coolant after a long drive (30 miles on the hiway). It used do that on short city trips (10-15 miles at 30-40mph) but changing to a new rad cap stopped it. it has no guage, but the idiot light for engine/overheat does not come on. no clue if it works or is hooked up, but car does not 'act' as if overheating (no pinging, no loss in power, etc)

there are no hydrocarbons in the cooling system, so the head gaskets are intack.

the 305 is coming out and going into my jeep this week, and in a bout 2 weeks a 400hp 350 is going into the camaro. except for belts, hoses, coolant all parts will be resused (new thermostats of course).

Any ideas?

My thoughts...coolant mix. it is now 90% water, so i have no way to check it's strength, but a 50/50 mix raises teh boilling point so if the mix was weak it might boil over....

thermostat sticking. both enigne intalls will be getting new 180F therms.

I'd like to try and identify what is at fault here so that the problem can be eliminated.

i will be fluching the 305 block and the rad while things are apart. i am tempted to pull the water pump and examine it...

any thoughts?