Go here, one of the few sites you can view and download Model Aircraft Design,
https://www.scribd.com/doc/55837270/...ircraft-Design
Chapter 16 deals with design and positioning of landing gear. Others who want to dabble in scratch building/design this book has a ton of information.
I have converted many trainers to tail draggers. In fact I encourage those who are building to go straight to a tail dragger and skip the trike gear altogether. Just about every high performance airframe that will follow your trainer will be a tail dragger anyhow so you may as well learn from the get go.
To convert a 40 size trainer to the tail dragger I like to improve the whole set up. I start with a good steer able tail wheel at least with a 1.5 diameter light weight wheel (hard foam Sullivan). For the mains I like to use the flat aluminium gear as its much more robust and easily mounted to the bottom of the airframe. Larger Sullivan foam main wheels in the 3 inch or larger makes for better control on grass. The landing gear and wheels should space the prop tip around 2.5 inches or better from the ground when held level. A little bit of 1/8 ply.
Cut and shape some 1/8 ply to match the taper at the underside rear of the fuselage. 3 to 4 inches in length depending on the tail wheel mounting bracket. Sand the outside edges round to match up to the shape of the fuselage bottom. Mark and remove the balsa bottom so the ply sits flush and glue in place. Mount the tail wheel then you will need to connect it to your rudder control. If the rudder extends all the way to the bottom of the fuselage you may be able to connect a small linkage direct from the rudder control horn to activate the steer able tailwheel. If the rudder ends above the elevator you will have to run another control rod from the rudder servo to the tail wheel. You need very little movement of the tail wheel to steer the model so the linkage should be in close at the activation point and as far out as possible on the tail wheel arm.
For the main gear the ply needs to be at least twice the width of the aluminums gear mounting area. Mount the gear in the middle of the plate then position in place on the bottom of the fuse. Cut and shape the edges like you did for the tail wheel. Remove the balsa and glue ply in place. You can reinforce from the inside with a bit of square or triangular stock.
If you carefully split the covering down the middle and peel it back before removing the balsa wood you might get lucky and be able to re-iron it back in place. I don’t worry to much as I cover the area with enough covering to make it fuel proof.
If you have flown the plane or run the engine, the oil, even after cleaning can make it difficult to get the covering to stick. I use trim solvent and wipe the old covering down before applying a patch over old covering. For best results round the corners of the patch to keep it from lifting. Apply the patch while trim solvent is still damp and smooth out patch. After the area has dried iron and shrink in place using the lowest heat to do the job.