I am putting this car on the track and I need a stiffer spring on right front. What stock spring will fit on this strut, like from a Tahoe or something like that

Perhaps a `99 Bonneville? . . . or a 2003 GP?

Might be a good idea to measure extended spring length, spring diameter, and the diameter of the cross-section of the spring bar near each end and in the middle. Take these measurements to the junkyard and find similar cars and measure their springs the same way. Might find some thicker ones out there that would fit the bill.