An Omega box would fit bit is a fair bit bigger and has a hydraulic clutch (not the end of the world though).
Best option IMHO would be a Carlton 2.0 box. Bit bigger than a Manta, but slightly better ratios.
A lot of the guys who have put a 1.8 box on a Redtop find they are a bit undergeared and rev high. Usable, but not great for cruising.