I started with products that are readily available at the local auto parts stores.  JB Weld is my go-to epoxy, so I stuck with their brand.
 
For the intake port, I looked at original JB Weld epoxy paste and an epoxy putty called JB Weld High Heat.  Original JB Weld is a two-part paste while the High Heat product is a two-part putty.  The original JB Weld is rated at 500°F continuous while the High Heat product is rated at 450°F continuous.

Here’s a shot of the original JB Weld.

For the exhaust port, I looked at a JB Weld product called “Extreme Heat”.  It’s a one-part paste.  It’s water base and contains steel fines, silicic acid, and sodium salt.  It’s rated at 2400°F continuous.

I allowed the products to cure for at least 48 hours after the initial application.  Then I blended the products into the adjacent aluminum to get a feel for the bond line.  I used abrasive porting rolls to do the blending.

Here is the original JB Weld after blending.  Pretty good edges but note the small holes.

Here’s a look at the Extreme Heat after blending.  Lots of holes but the edges look OK.

This shows how the High Heat held up to the saw.  It showed a very tight joint with no evidence of separation.

Because I was concerned about water solubility with the Extreme Heat product, I placed half of the specimen in a jar of water.  Within 15 minutes the entire layer of cured paste was gone.  It totally dissolved.

While there is little concern over water in the exhaust port, I am a BIG fan of a nice hot, soapy, water bath prior to assembly.  I want ALL the grit out before I put it together.  Port mods require grinding, and this cured product is like rock.  Clean-up after the port work will be mandatory and a filler that dissolves in water just won’t cut it.  Things were looking bleak for the Extreme Heat filler.

Just for info, I submerged the two epoxy specimens in water and left them over-night.  It didn’t affect the epoxy.  Time to lite-off the BBQ.

I baked all the specimens in my BBQ at 450°F for four hours, then I removed the two epoxy specimens and left the Extreme Heat specimen in the cooker.  I raised the temp to maximum and monitored the surface temperature of the specimen every hour.  

My BBQ maxes out at around 575°F but I figured the specimen would be significantly hotter since it was directly above the burner.  I would shoot the specimen with my infrared thermometer each hour.  I measured temps around 600°F.  Its sort of a moving target because as soon as you lift the cover the specimen is exposed to ambient air and the temp immediately starts to plunge.  I got it as hot as I could without putting a flame directly on it.

The specimens looked pretty good after the bake-off, but the slight separation line was starting to become a bit more evident on the original JB Weld specimen.  Otherwise it looked solid.  None of the specimens showed any sign of imminent failure.  They were all hard as rock and seemed to be holding up well.  The only significant visual change was the epoxies were getting darker in color.

To see how the heat affected the water solubility, I threw them back in a jar for another soak.  The Extreme Heat product now showed no evidence of dissolving, but it swelled up.  It was lookin worse and worse for Extreme Heat.  It’s just doesn’t look like a good product to use in any sort of uncaptured application.  I think it would be OK in a threaded hole, or as a sort of filler/sealant between say my special exhaust insert and the face of the port.  But anywhere it isn’t captured, I don’t trust it.

I did another test on the Extreme Heat to see if it becomes insoluble at lower temps.  I baked a specimen at 250°F for two hours, then placed it in a jar of water.  It didn’t dissolve, and it was still hard as rock after the soak, but when I pressed on it with my thump it popped right off the test angle.  I’m not likin that.  Here’s how the stuff looked.  I’m not likin the gas pockets.

Here is an example of an adhesion/tensile test specimen.  I want to see the bond between the nut and the epoxy hold up and the epoxy fail in the center of the joint.  That way I know the adhesion between the epoxy and the port wall should be good enough to keep the filler material in place.