1 - According to ASME IX for qualification a welding procedure, pipe qualifies plate and vice versa. So, you just need to use a pipe or plate which its thickness falls to the range of SWPS.
2 – according to ASME IX (welder qualification), any F-No. 6 qualify all F-No. 6 and any base metal P-No. 1 through P-No. 11 qualify P-No. 1 through P-No. 11
Both ER 308L and ER 70S-2 fall to F-No. 6, so your welder is qualified.
However, I feel that something is wrong.
Probability to make a weld defect is lower when a 1/8” plate is welded; comparing to 1.5” one for demonstration and doing radiography test. Therefore everybody prefer to use a 1/8” plate!!!!!
If your 308 WPQ was performed with backing gas and your SWP's does NOT use backing gas, then the welder is not qualified for the root pass. Other than hat, you should be fine provided all other variables are within the production ranges to be used for carbon steel.
G Austin