Biggest issue is to make sure that the outside diameter of the cable jacket is small enough to fit in the connectors on the ends.
L14-30P and C plugs and connectors....... Bryant are .465 inch to 1.150 inch diameter while P&S is .325 inch to 1.150 inch diameter.
The Southwire Viper cord seems from my research to be the largest diameter cable (thicker jacket) of any I have found. 8/5 is 1.015 inch which just falls inside the above mentioned connectors. Have not looked at Leviton, Hubbell, and Cooper/Arrow Hart plug and connector specifications but I suspect they are probably about the same. I've not had any luck finding out the minimum and maximum wire sizes but I imagine #8 is within the range of a L14-30 plug or connector.
The reason I have found out so much is this thread got me to thinking about Craigslist and I did a search today for "SOOW cord" and WOW!!!! what comes up is a week old ad for "65 ft of 8/4 SOOW cord" for a hundred dollar bill. Well, it turned out to be two pieces of Carol 8/4 SOOW, one piece was 37 ft and the other was just shy of 50 ft. This worked out to less than $1.18 a foot but I have to buy two connectors and plugs rather than one of each, but what I found on the net was $2.15 up to $4+ a foot and then shipping on top of that.
There are no UL markings on the cord, but I found looking on the net that apparently SOOW is offered both UL and non-UL with the UL being quite a bit more expensive. Since its good Carol brand and its for a Generator, I'm not real concerned. I think part of the difference may be the jacket thickness. The specs on the non-UL cable generally reflect (for 8/4) a diameter of about .750" give or take a little, while UL cord generally seems to have a diameter of about .950"
The commercially available cords are all 10 gauge and have the molded plastic ends that use the plastic to apply pressure to the connectors (female end) to grip the male plug. I do not like this and prefer to have a real connector end with double wipe connectors that are spring type. I also prefer a real rubber cord and not that plastic stuff.
I have connectors and plugs coming, and I have the inlet plug box, and PVC conduit to run under the back porch and 10/3 w/grd Romex to run thru the conduit and under the house. Also have the breaker, just need the Square D QO type S NEMA type 1 panel interlock for 150/225 mains. Gotta order it yet. Best I've found it is $73.
Ouch!!!! this generator is costing alot more than just the price of the generator.
Charles