I am building an airboat, the prop shaft will be above the engine
driven by belts. I was planning on using a 2' long 1.5" shaft
supported by 2 1.5" pillow block bearings and then making a hub to
mount the prop to on the end of the shaft. The side load of the
bearings would come from the prop pushing the shaft. I priced the
tapered roller pillow block bearings and they are $350.00 each, not an
option. The other idea I thought of was using a 1 piece rear axleshaft
from a pick-up as the prop shaft. Make an adapter for the prop to
hubface and the tapered rollerbearing on the other side should take
the load. Mount the shaft with 2 pillow block bearings. The boat will
weigh about 1500lbs and we are using a stock chev 350 at 2500-3000rpm
max.

OK so you will have way more thrust load than pillow blocks normally see,
but maybe comparable to truck cornering forces, so I like the idea of
adapting an axleshaft. But, cornering forces are only intermittent. So I
think it might be worth doing some simple design sums. Do you have a prop in
mind; if so, presumably you can find out the thrust force at typical revs?
Even if you don't have bearing catalogues to hand, post the load and speed
here, and someone will help you size your thrust bearing for a given service
life.