The key to deciding on this is your words "do not want to spend much money on this".

Just keep your 390 and when you get some money, start with the exhaust and add things as you can afford them. After the exhaust, you can look into a 3.98 crank with pistons, then heads, cam and intake. When and if you want to put in a stroker kit, have the cylinders checked to see how far you can bore safely to increase your cubic inches. Remember 428 cranks (3.98)are externally balanced. A 410 (torque) is a big improvement over a 390 when a heavy vehicle is concerned.