I'm not sure you'd even have to change the Springs, the OHC engine is a pretty heavy engine and the SBC is much lighter than a Pontiac V8.
I believe the OHC is around 460 lbs without the bell housing and flywheel, the SBC is around 575
Install an aluminum intake, 540 lbs, use an aluminum SBC bell and headers instead of Iron manifolds on the SBC and the weight should be close enough to not be a problem.
Plus the SBC fits further back and lower in the frame than the OHC.