the lift is the important factor for spring selection. any good spring company will be able to tell you the maximum list their springs can handle. its essentially:
(fully extended height) - (fully compressed height plus 0.01" per coil) = max usable height
but there are a few other minor factors like seat pressure and whatnot.
i've talked with another RS owner who built his SOHC motor using i believe mustang cobra type r springs because they were the same dimensions as the SOHC springs.