Nose is very heavy due to change.
What is the best weight to use to change?

Lead, wood, ect.</p>

lead

It's quite odd to see the change from tricycle result in a very nose heavy model. You might want to study your main gear. It doesn't have to be heavier than the mains you removed. Since the movement of the mains from behind the CG to in front is usually about equal in distance, there isn't any real increase from the move. The difference comes from removing a nose wheel and replacing it with a tail wheel. And that usually doesn't ruin the balance because the nose wheel's fairly close to the CG and the tail wheel is about as far away as you can get.

But to answer your question, weight is weight. Choose what best suits. Lead is usually chosen because there isn't much room in the tail and to use the least amount of dead weight you place it as far aft as possible. So you use what comes in the smallest size.

What is your new convention gear made of? Where is it attached relative to the leading edge of the wing?

I take a bunch of pennies and see how much weight I am needing by adding them a few at a time.

As daRock said, the conversion shouldn't shift much weight towards the nose. You're removing a large nose wheel and adding a tiny wheel way out on the tail. Before you add anything try moving the battery and receiver back. Anymore you won't gain much with the receiver . . . but it all helps.

Sheet lead between the tailwheel bracket and the fuselage is a good spot. The further back the less you'll need. And lighter always flies better.