I would check
www.painless.com if the truck aready has a wiring harness from painless. I installed one 10 years ago and my wires are individually labled. If you can find the part number for the kit they have PDF manuals on line. Many times, with the newer wiring, the wire sizing is designed for an HEI. That depends on the kit and the age. You may not need the relay. Mine has not had one in all these years. If I was unsure I would add the relay. The painless folks are great to deal with and will probably answer your question if you call them. The following is from my painless manual. I have never had any problems with the wiring or kit and I don't have relay. Disclaimer : I am not implying anybody else is incorrect and I am not an expert.
7.3.4 If you are using a Ballast Resistor, mount it away
from other wiring or hoses. The Ballast Resistor gets
very hot during operation. Connect ENGINE
SECTION A wire #920 (pnk) to one end of the Ballast
Resistor. Connect the other end of the Ballast Resistor
to the Ignition Coil B+ terminal with 14-gauge wire
(you may have enough pink wire left over to
accomplish this). If you are not using a Ballast
Resistor, connect wire #920 directly to the Ignition
Coil B+ terminal.
Note: The ballast resistor has been deleted from
this kit due to lack of consumer usage. If one is
needed in your application, please call Painless
Performance at 800-423-9696 for assistance.
Important Note! For HEI systems route wire
#920 (pnk) to the Distributor and attach it to
the terminal labeled BAT. No Ballast Resistor
is required.