A long time ago (late 70s) my Dad help me build a P-63 King Cobra C/L. This was not a big kit and was rated for a .09 - .25 engine. We growing up with a lot of brothers and sisters (my parents were foster parents -- way cool, but not for me) we recyled everthing. So my beautiful Profile P-63 flying on 50' wires was powered by an old steel sleaved K&B .19 Green head. This plane took about 8 16 Penny nails in the tail to balance it. It flew rock solid and would even wing-over (C/L guys know what I'm talking about) but don't pull to many Gs on anything. She would actually nearly stall and wobble through at the bottom of a loop. After getting a job delivering papers, I bought a brand new Fox 0.15 with the offset glow. More power and at least a pound lighter (maybe stretching it a bit) but what a difference.

My earlier post on my 9 lbs Supersportster on how great she fly's is true "Heavy plane can fly very good."

But to the point:
--Build light (but you don't have to go crazy like cuting a hundred lightening holes in a kit that doesn't call for them)
--Build strong (this means re-enforce where appropriate and the the correct materials don't skimp on structure)
--Finishing can add as much as 25% to your model!
--Its better to come close to the rated weight with an engine in the specified range than to over power and add lead (most of the time, but a lot of exception to this one -- mostly applies to sport and trainer types -- with warbirds we generally are about 1/2 ton heavier than rated -- I mean sometimes flying at nearly 50oz of wing loading. So what's another 20 oz if you can get another horse under the cowl)
--If its 10%-20% over weight with a strong engine it will probably fly great, so great you'll never even think about weight again with this plane.
--If its more than that -- well anything goes. It could be a great flyer, a good flyer, or a brick (warbird = 2x4, lighter than a brick with better aerodynamics).

So just build, build, build, build. Don't spend to much time wondering if you should cut 5 or 10 lightening holes in that 1/8th horizontal stabalizer. Use your head when re-enforcing (1 oz fiber glass and paint should be skipped on .20 size models). Fly the heck out of what you build even if it flys like a warbird (mine looks soooo cool on a high speed with her gears up). If it is to heavy to fly safely than pull your radio gear and your engine. Add a bunch of scale details and dummy stuff. Then hang it from the ceiling so you can admire your work on those raining days. Beside, hanging a beautiful non-flying model in your family room lets the rest of the family members share in the craftmenship of your art and passion.