straight six

Own a 1998 Grand Cherokee Laredo. Would like accurate information regarding swapping out my straight six, which with 184,000 kms does not burn a stick of oil. Just replaced the original spark plugs. Very strong reliable engine. However, thinking about changing out to either a 318 or 360 which I think were the available engine options. Is this a fairly simple swap or do I get into fabricating engine mounts, changing transmission, tcase and other d/line components ? Or, other suggestions.