The two items on BLARS are dated 1782 and 1785, so to me he seems to have been a bit young for the references to be your James. But he was a saddler so I wonder if it might be his father and he carried on in the same line of work.
It doesn't imply to me that he was a Quaker, merely that he was doing work for the Quakers. Was your James married in the established church and were his children baptised in the established church? If so then it definitely doesn't appear that he was a Quaker.
But if he was a Quaker then you're lucky as Quaker records are extremely good.
If you search for Quakers on Google there are plenty of sites which explain Quakers and their record keeping.
The 1841 census merely says that he was born in county, ie Bedfordshire, not the town of Bedford.
Your James was buried at Dunstable on 4 Dec 1851 aged 78, but there is also another James Custance buried at Dunstable on 29 Jan 1811 aged 64. There was also a Frances Custance age 72 buried in Dunstable on 6 Jul 1821. The only Custance burials in Beds were in Dunstable, the first being in 1808, so it looks as though they were from out of county (or Quakers!). I can't see a marriage on the IGI between James Custance and Frances, but they baptised a daughter, Fanny, in Dunstable in 1787, the earliest Custance reference in Beds.
Regards
David