Unless You want somebody to attempt to machine those spring seats with the heads on, which could be accomplished w/the right tooling & caution/experience, then it defeats the "cam/intake/carb" only swap. Retainer's not the problem, it's the heads. Since it IS the smallest of the Comp solids, I'd do exactly what RRR said, that or the "0249"/340HP DC/MP spring. The 901 is the std. recommended spring, & and NO You cannot just use the outers from the duals, in case You were wondering....