Here's a question for carpenters.?

The trouble with that is it's going to twist, warp and look like crap no matter how you finish or seal it. Southern Yellow pine is quick growth timber and nothing will prevent it from doing it's thing. Fir would be better but only marginally so.Best bet would be to look around to see if you can find someone selling "Barn" lumber. It's salvaged from old barns is generally well aged and won't warp, twist and crack on you. Sand and finish and it should look good. Another possibility, since you're not hauling a load is engineered or composite decking if you can find some at a reasonable price.

Composite is definitely the way to go for longevity. I replaced wood slats on an outdoor bench 15 years ago and it still looks new.

Yes, it's expensive, but I think you need to amortize the cost vs future maintenance.

Also, take a look in the "free" section in your local craigslist, you never know if someone is tossing some materials and just want someone to take it away. Free is good!

syp or even spf grades won’t due well outdoors without being heavily treated and even then may need replacing soon

treated lumber can be stained, just use a deck stain and deck cleaning procedures

if you are just using construction 2x material they won’t be smooth, will have tear out around the knots and a sanding sealer will need to be used to keep any stain from looking blotchy

another problem is they are very soft woods and easily dented or scratched

white oak is better outdoors than red oak and costs a little less

any hardwood that performs well outdoors will be more expensive than common oak

SYP is not as bad as they all are saying. It will be bolted down in 4 spots. Stained and treated on all 4 sides. You will be good. Most trucks with wood floor are made to be replaced anyway. So if its not in your budget this year, maybe you will change them out in a few years from now. It will look fine. You can look at reclaimed wood. But it will take work to get the old wood clean and straight. I would use it. You can always change it.

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there is nothing wrong in using treated or wolmanized lumber and you need to buy that wood and let it dry in your house or in the garage === a 7 & 1/2 foot width for your truck bed is going to be too wide and it will not look right .... I suggest that you carry your tape and do some serious measuring on truck beds ..... same goes for the bed sides of the 1&1/2 angle iron look and check that the wood is up and clear clear of the iron .... forget the yellow pine as it will warp and twist in the rain and sun ........ the SOuthern Y. pine is rarely dried and as much as you try to protect that wood it will not make a difference ............