Alvin, I have no experience of 'aftermarket' heaters,I've got a perfect original fresh air unit but due to lack of available serviceable heater cores I opted for the old British favourite.
My friend fitted one in his '52 and it was toasty!
An
Austin Mini unit.
It's compact,self contained,cheap and best of all plentiful(at least over here).
They do turn up on ebay 'over the pond' occasionally.
Just my 0.02c.
