If it is strictly a plains game hunt, you really don't need a cannon. I returned this April from RSA and took a 6.5 Creedmoor and 300 Win Mag. The Win Mag was used on an eland, but I have killed them before with 30.06 several times. The creedmoor handled everything from kudu, to wildebeest to spring buck and bushpig. I have used .270 Win and 30.06 and 300 WSM on many other plain game hunts. My boys shot .257 Roberts when they were smaller.
Most shots will be well under 200 yards, and often less than 100 yards. Use quality heavy for caliber bullets and don't sweat the actual caliber. Take whatever "deer rifle" you already shoot well.
When going for the dangerous game my advice is quite different.
Bill