Hey there, rykoala - another fan of the Marauder! I got the book (Atomic Zombie's Bicycle Builder's Bonanza
off of Amazon a month or two ago; have been drooling over the Marauder ever since. It's a very simple design that could easily be made from the two donor bikes you described and a few feet of new steel.
For the seat, I'd recommend going with the simple foam-over-plywood he's used. It's a very reclined recumbent position with your weight distributed widely, so I'd imagine this would have plenty of comfort and be very simple to build.
If you're looking to build one, I'd highly recommend his book (link above) - he has very detailed building instructions, photos, tips, etc. This would provide all of the information you need to find dimensions, and give you and the builder something clear to work from.
With the builder living so far away, you'll definately want to be prepared... these things always take longer then you'd expect. Come up with a general plan, and then start cutting up the donor bikes and piecing it together on the floor until you get a general shape. Take some measurements and try to make it the correct size, then add on some length - you can always take it off later when the bike is built and you have a chance to ride it.
In his plans, the rear triangle is constructed from a rigid front mountain bike fork, some chainstays, and exta metal. It works great, but is complicated and difficult to keep everything in alignment - it has to be perfect and could eat up a lot of time with your welder. If possible, see if you can use the complete rear triangle from that mountain bike... it would save a lot of time and frustration.
I'd like to build one this fall, but set up for touring with underseat steering and an emphasis on comfort over all out speed. It's not a typical touring design, but would be very comfortable, reliable, stable, fast, and relatively simple to build. Check it out with a trailer
- perfect for touring.
Keep us posted on the building, and PM or email me
if you want to discuss the details.