If you're giving out the Sole Prop (SP) EIN on a W-9, then the checks coming in from doing that work should be made out to the SP.
That said, I'm not seeing a way that you can participate in a solo 401k under your spouse's SP. If you were a partner in the business, you could participate, but then it isn't a SP anymore. If you weren't a partner in the business, you would need to be an employee of the business, in which case I would expect to see a W-2 for your work.
Maybe
@SeattleCPA or another expert knows something I'm missing? While I always suggest these retirement accounts, I do encourage clients to discuss with their provider for the questions on eligibility and contribution amounts.