Bob,
The Ron Davis radiator tube and fin portion is thicker than the stock radiator. In order to install it you need to make 5/8 inch additional clearance to the fan housing. This can be done two ways:
1. Trim the fan housing. OR 2. Add a 5/8 inch spacer bar to set the fan housing away from the radiator.
I installed a DeWitts radiator using option 2 and the Ron Davis radiator using option 1. (Two different cars.) Here is a picture of the Dewitts using option 2.
I prefer option 1. Trimming the fan shroud took about 15 minutes and is impossible to detect once the fan shroud is installed in the car.
By the way, there is difference between the DeWitts and the Ron Davis units. The DeWitts unit is thicker that the stock unit at the top edge where the rubber mounting blocks support it, requiring the rubber blocks to be trimmed. The Ron Davis radiator fits properly in the support blocks without trimming.
Good luck,
Jim