I would like to give a go at building my own headers so my question is how important is it to have the tubes close to the same length? I have a 2x3 tube chassis frame and have the motor centered and will have a front coil over suspension and rack steering so I have lots of room. I will be using 2 1/8" primaries with 4" collectors.

It really helps to evacuate the cyls... you might want
to go bigger on the collector... if you want PM me
and I can run your spec through my header program

What would be the optimum size? This is for a 493 stroker, tunnel ram,2x750 DPs.

Hedman has kits for tube cars that are very inexpensive..........Buy the BBC kit and get the flanges elsewhere.
Like here........... http://spdexhaust.com/
That may be the cheapest route and the kits are virtually fool proof.
Pre bent tubes....just assemble , make a few sparks and you're done.

Max RPM will be 6200-6500 so I'm guessing around 32"-34" for the primaries. Would this be close Mike?

I built mine with premade stubs and collectors and a box of bends. There not equal length and still picked it up a tenth in the 1/8 over the fenderwell headers I had.

Thanks for the input guys.