I would avoid Poplar because it is to soft to last very long and look good.
If given the choice between red and white oak white is better. Red oak is not very rot resistant, white oak is very rot resistant.
I know there are many suitable woods for a flat bed, personally I will use quarter sawn white oak for the flat bed on my 48 2 ton.
Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-