The start of this bike can be found here: http://endless-sphere.com/forums/viewtopic.php?f=3&t=18157
I wanted a bike that was slightly longer and lower, but had full suspension front and rear. I wondered why few had put a rear-sus arm from an MTB onto a hardtail MTB by bolting the pedal-axle (Bottom-Bracket, BB) into the drop-outs. I tried it, and I like the result.
My feet are now flat on the ground, the seating position is slightly more semi-recumbent (like a beach cruiser, rather than leaning forward like a road bike) and the combination of fat tires and FS make the bike handle rough sidewalks and potholes in the street very well.
The oiriginal seat and shock-absorber mounts were made fast and ugly. Once I determined that I liked it, that there were no major issues when configured this way, and I also located the position where I most liked the seat, I began pondering a more permanent arrangement of parts.
I wanted to open up the rear frame space for some type of non-hub motor with a Left-Side-Drive (LSD), but, that is at least a month away (or more). The mods so far have been accomplished with a vice, hacksaw, drill, and files.
The seat-post will be a fork (possibly from a 12" childs bike), to allow the shock to pass through. The seat-post mount shown is a copper plumbing-T with a brass seat from a kitchen faucet (part# 0296, "American Standard" brand). The outside diameter of the cylindrical brass valve seat was 0.875". The holes in it were there from the factory (all holes approx 0.33")
In Plumbing terms, the port that the brass valve-seat was able to slide into is called a standard ___ copper-tubing size. The main 2 ports were 1.0" because my frame has one-inch diameter tubing there (other diameters are available). Though, I did cut half of its diameter away so I could pop it on from the side, and then clamp it down with two stainless steel hose-clamps.
I like the way the copper looks, but if you want to save a few dollars, use a cast plumbing T (brass or iron), and a section of plumbing pipe with threaded ends. Cut the pipe in half, thread one of the stubs into the center of the T, smash the pipe stub flat, and drill a hole.
Once you locate where you want the frame additions, a more professional-looking arrangement (that is still affordable) would be to cut out some joints from another frame to make lugs. Then braze them onto your frame with a MAPP-torch (of course, study first and practice on junk). The lugs in the pic below are curved so stress is spread out (its not just for decoration)
If your frame tubing is not round, perhaps rub a waxy lip balm onto the frame, and fill the void with JB-Weld metal-epoxy (which will adhere to the inside of the fitting). Once dry, cut and file away the excess, then clamp it down. This will spread out the clamping loads, so you can get a good grip without crimping the frame tubing. The epoxy forms a perfect fit to oddly-shaped tubing, and the wax prevents it from sticking to the frame.
Also, if using U-bolts as a clamp (as I did on the shock mount), you might consider making a clamping saddle from a section of steel pipe that has been cut lengthwise. If anyone doesnt like this plan, just wait 20 minutes, it will change.