thanks, Pete, Chris, and Colin.
I traded msgs with Demisterman.co.uk. When I showed them my matrix sketch and dimensions, they replied that one of the matrix dimensions on their website is incorrect, and that the center distance of the tubes is a little more narrow than stock. Otherwise it would fit, and "others haven't complained", as we are often told.
I chose to have a local radiator shop repair my old original heater matrix. They took off the plain end cap, rodded the tubes, flushed well resoldered the end cap, straightened the fins and reformed a distorted tube bead. I did a little pressure check in the sink with a ball pump. It seems like the leak is fixed.
I am still unsure about getting a reliable seal between the valve, plain pipe and bracket. The existing seal appears to have been made by a triangular cross section rubber "O" ring and a paper dished washer. Do you suppose an ordinary "O" ring was distorted under the clamping force to form the triangular cross section? I am thinking that a rather softer durometer will allow the required distortion to for the seal. McMaster-Carr shows soft silicone high temp "O" rings, and water, steam and ethylene glycol resistant "O" rings. I need to make a wise choice of dimensions. Does anyone have a record of their successful choice here?