Great question! I'm not an aircraft designer so I don't know the correct answer, but I hope this thread brings in some people that know what they're talking about.

My feeling is that you're engine weighs too much.

Enlarging the plans for a typical trainer x 2 gives results that are more than twice your specifications of 6 foot length and 1500 sq inch wing, and yet your engine is nearly 9 pounds and the average trainer engine weighs about 1 pound.

For example, an LT-40 with an OS 46AX is an airplane with a 1 pound engine, a little over 1 HP, with a wing area of 900 sq inches, weighs about 6 pounds with a length of 56"

If you enlarged the LT-40 at twice the size with a 9 pound engine, maintaining CG would require many pounds of weight at the tail, and this is the issue. You'd go from what should be a 12 pound airplane at double the size to as much as 20 pounds due to beefing up the plans.

I think it would fly, but probably not very well. I think it would need to fly very fast to stay in the air, and I don't know if the typical trainer would be up to the task.

I assume this the only engine you have available, I hope you can make it work and hopefully someone will step in and say how pathetically wrong assessment is.

Can you strip the engine and make it lighter?

Once again, great question! I hope it works out for you.