[gumby79]

Galvanic action

Adding brass/copper to your systom could cause a corrosion problem. Theas 2 meterals were removed from the cooling system with the introduction of Al rad. It took the manufacturers a few years to figure it out . How ling wi it take to decime a problem? Dono... 3 years on my '79K-20 with a 4core Al rad. and copper/brass heater core no coolent related trouble.
---
An idea to make your sealed chamber.
Braze up your shape (3sides 4th is the trans case). Use some gas tank sealer epoxy.
This will remove the air/water tight requirements, making your welds structural only reducing the likelihood of a contamination leek. Oil has soked into the metal and will surface when heated.
---
add a trans temp Gage.
Add codes for cooling function, to allow lower oil weight without the threat of over heat failure in summer.
---
Not enough rum time to justify this mod???
Add a pump .
Im shure thar you have already exploited the low hanging fruit (block heater). With a relatively short commute lack of heat bulid up is what your trying to fight. Set the circ pump to come on after the motor is warmed(on the plug)
----
Alternative to a jacketed heater.
Use a tube/fin or stacked plate type cooler were you want the jacket then insolate . Option to improve heat transfer fill the fins with Al chips/shavings/ powder improving the contact patch with the cooler and trans case as air space only transfer IR heat (slower)

Well enough ideas for now.

Dream Big Chisel Down To Reality