Mad Turk,
Depends what you intend to do as for what size tinny you will want - if you are ging to move from place to place and have to reload the boat, motor etc back onto the truck, i personally wouldn't go any bigger than a 12 footer with a 15hp - otherwise its a lot of work lugging the gear up and down.
if you are going to be camped in one spot for your trip (as our trips north tend to be) i'd go a 3.9m with a 25 - on the basis you only have to move it once up and once down. - take some of those clamp on wheels for dragging the boat up the beach at the end of the day too (make sure they are air filled tyres so you can drop the pressure) - much better than dragging or complete lifting the boat up above the high tide mark at the end of the day . (or if you are staying in weipa - possibly a fold up trailer (you may even be able to hire a trailer in weipa)
i've seen a 4.2 wide body dory on the top of a nissan ute - but the weight in that would be getting up there - personally i don't think that you should go over 75-100kg - with 100kg at the upper end of what is generally considered safe to have up top (bear in mind i have a ute with a purpose built boat rack too - keeps the weight a bit lower than on a roof rack on top of a wagon.)
you mention weipa - if you are staying there for your trip - consider that the rivers are very big - you can head up the embley and after 15minutes heading upriver the river banks still disappear over the horizon straight ahead - and they do get very rough - particularly when the wind is against the tide. so keep that in mind when choosing your boat
remember the swamp geckos are pretty big too - so a small low sided tinny is not a good idea - i personally wouldn't take a punt to use at weipa either due to both the crocs and how rough the rivers can get. consider whether sending a bigger boat up by barge from cairns is an option - depending on how big the boat is - would cost less than a grand each way
My brother is in weipa and the fishing options we have when we head up there and use his 16 footer centre console are exponentionally better than if we only had a car topper. - if this is a once in a lifetime trip for you - the cost of barging a bigger boat up there may be worth it?????
a last tip - check out mapoon an hours north of weipa - there's a camp ground there with cold shower and toilets - you won't be dissappointed
- we've fished from there very successfully with a 12 car topper- but again much better out of the big boat - just watch the alcohol restrictions.
Moffy