Not too sure what the BTU cap of the original 2.6 rad was, but I think the intake and outlets are on the wrong sides? The GM rad was too wide to fit into the space in the Mitsu.
Keep the mech fan if you're towing. At worst mine was cycling on about 25% of the time on the up-hills on the freeways in 100 deg F of more weather at elevation in ID/CO/UT. I don't have a fan shroud anymore, so it was originally a cause for concern.
I also have a 17' Wellcraft boat and trailer, and I doubt there will be any concerns there either. The weight's certainly less significant since you're putting an engine into a vehicle that's about 1,000 lbs less in weight than the vehicle it came out of.
You'll love the 4.3 for towing!