A crew cab is doable but to qualify there are regulations of how deep the boot space has to be and you may have to put in a rear bulkhead.
Just fill out the forms and will need an engineer to fill out one of the forms, it's a report but has to be on the official form not a letter from engineer.
Get it weighed, doe'd and bring forms with money to tax it to motor tax office and away you go.
You can make anything commercial once you meet all the criteria.
I know of a supra and Mitsubishi gto that have been converted.