The 510 Realm

i probably will just buy one, but i thought i could cheap out with a non-standard that might be close enough, without a lot of work to fit it in. i was hoping someone already made this mistake (i mean had this idea).

anybody know (or tried) if a 620 or 710 (p/u) header will fit into a 510 wagon w/an L20B? i have the option of clamp fit (3 pcs), or with a collector flange. welding is not a problem, bends may be. thanks.