I would just like to again say thank you so much for all the comments and suggestions. It is most appreciated!

The problem has been rectified by the replacement of the distributor. The failed component was the armature assembly (magnetic pick-up) of the Hall Effect distributor. The armature can be purchased separately but it involves the complete disassembly of the distributor for installation. I found the purchase and installation of a rebuilt distributor to be much less bother. Re-man distributors for this truck are available from UAP/NAPA

Neither the DFR fuel filter, the TFI module nor the ignition coil were at fault. I did not initially suspect an ignition problem because the engine would start and run for several minutes before shutting down and swapping out the coil and ignition modules with serviceable spares seemed to have no effect.

Even the failed armature would provide an initial spark during cranking before ceasing to function all together. I had since read about an identical fault with an 1986 Ford F150 fitted with the 300cu. in-line six cylinder engine so I suppose the issue is not rare.

Anyone else experiencing sudden engine shut-down or a 'no-spark' condition after initial cranking spark on these trucks might first want to consider a failed distributor armature...