[V8] Heater core BTDT?

[urq]

... got the heater box out of the car the other day ... yes, remove the hood
... the box is considerably bigger than that in the old '88 5kCSQ.  I also
had to loosen the hose clamp on the refrigerant line above the driver's
footwell to get the line out of the way ... engine loom was also a bit
difficult to get around.  

So ... I've got the new blower motor and heater core installed and the box
is back together ... and I'm wondering what sort of material I should use to
fill up the hole at the fitting end of the core.  The original had some sort
of rubbery material, but it seemed a bit more pliable than RTV Silicone.
The manual doesn't give details ... so, what have others who've BTDT used?
I'm thinking I'll just run a bread of silicone around the edges to seal it
up, but without filling the entire area with the stuff.  

She's going back together now ... you should see all the stuff I ended up
pulling for all my little clean ups and updates ... 

Ah, yes, one thing I want to check tonight is the illumination of the HVAC
vents ... tried this before, but it won't work without the instrument
cluster installed!

Steve B
San Jose, CA (USA)