FPHEs are quickly becoming a standard component in most VO conversions due to their efficiency at transferring heat from coolant to VO fuel. Due to this efficiency they are very compact units which it is normally very easy to install under the hood.
Two basic installation configurations have been used with success one that places the FPHE just prior (according to fuel flow) to the VO filter and one which places it just prior to the IP. It should be noted that if the second configuration is used a small in-line "chunk catcher" filter capable of withstanding temps of up to 200°F should be installed between it and the IP. Check the component vendor section for links to these. You will find links to FPHE vendors there as well.
Because FPHEs are very efficient (if installed correctly) bigger is not better in most cases. A single 7.5"x3"x1.5" FPHE is usually sufficient for most passenger cars or "light trucks".
It is best to plumb the FPHE with hot coolant flowing into B1 and out B2 and cold fuel flowing into A2 and out A1. However..this only works best if you plan to mount the FPHE on its "back" with all fittings pointing up (as illustrated below) so any bubbles of air that flow into the FPHE can easily escape.
Mounting in this orientation allows cross flow of the coolant and fuel...which is the most efficient flow pattern possible.
Here's an example of cross flow:
If the FPHE must be mounted in a vertical orientation (as illustrated below) then the flows of both fuel and coolant must be from the bottom ports to the top ports or any bubbles of air entering the FPHE passages may become trapped and lower the coolant/fuel interface area...lowering the efficiency of the FPHE...sometimes dramatically.
Mounting the FPHE in any other orientation is not recommended however...if there is no other option a FPHE can be mounted "sideways" with the VO ports on the "up" side. Similar to the image below...but not quite.. since if you look closely you will notice the coolant ports are on the "up" side.
Adding a 1/2" of foam insulation to all sides of an FPHE increases their efficiency by 10-15%.
Links: For more info on plumbing coolant circuits go HERE.