As you may know, what you have to legally declare depends upon the role being applied for. The vast majority of roles in financial services only require a basic DBS check and you do NOT have to disclose any SPENT convictions. From your description, your conviction would be deemed spent in the UK.
Where you would legally have to disclose spent convictions are for roles that require a Standard DBS check (exempted from the Rehabilitation of Offenders Act). These are roles that used to fall under the regulators 'Approved Persons regime' - roles that have a significant influence over a Bank's (or other types of firms) operation and clients/customers. These roles required regulator (FCA) approval. (The new Senior Managers Regime replaces a lot of this with a form of self-certification by the Banks but the principles and guiding law still applies)
Unlock provides some more detail here: Unlock-ROA-FinServ
I would call the FCA and ask for their guidance, albeit with anonymous names for yourself and the Bank concerned. Alternatively, share the general nature of the role and I can give some guidance on the likelihood of the role requiring a standard check.
One final comment - if you are a non-UK citizen and the offence was in the US, the details would not have been shared with the UK authorities. Even if you are a UK citizen, the sharing of overseas convictions is not automatic so given the time period, the conviction may not be on the UK PNC - you could find out for certain by making a Subject Matter request (see the Unlock Info Hub for details of how to do this). And if it's not on the PNC, it won't appear on any DBS check at whatever level..