Aa a "rule of thumb" 1 HP per cubic inch is a good target for a performance build... that's well put together with a warm cam. With a heavy vehicle, you would probably be happier with a more moderatly cammed "torque build" in the 250-275 HP range with a 318. With Magnum or EQ heads it would make a good engine for a Ramcharger... but miss your power target by a lot.

For maximum power on a budget, I think the best approach is to start with a 360 magnum core as several have posted. Depending on condition, you can re-furbish the heads or replace with EQ and build from there. If you found one that didn't need rebuilt, you could make 300 HP easy with a cam and manifold swap.

On the transmission, I do not know what the RC had from the factory. I agree a 904 would work... but a readily available 727 would hold up better in a heavy vehicle, especially if you get power and torque up with a stronger engine.