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Ok so a public library in my area got 3d printer and Its available to use. I'll be going up there to use it next saturday and I'm going to try and prototype a few redesigned parts.

Since these parts are going to be plastic, It will be easier for parts that have threaded holes to just be a hole and let the screw im using create the threads.

What size holes should I use for 4-40 and 8-32. Right now I'm thinking 0.09" and 0.13" for the holes and I should be able to make the screw go in with a little force.

The 3d printer I'll be using is a makerbot 2 and the parts will be ABS plastic.

hopefully This will serve as good prototyping method for parts I'd like to have machined.

The first part on my list is going to be the rear bulkhead. It is really blockly but hopefully that will lower machining costs in the future.

The drill size to tap a 4-40 is a #43 drill bit (0.089"), and 8-32 is a #29 (0.136").
http://www.boltdepot.com/fastener-information/US-Tap-Drill-Size.aspx
You can always use a tap, cutting or roll-forming (tighter hole). A little small is OK, but if the material is too hard you can crack it, and if the material is too soft you could cause it to deform in other areas.

I've still got a few days and I'm good with the parts I want to do, any requests for a part you'd like to see done.

Here is what im gonna try to make for now

I think a spacer to fill up the void between the 3/8x5/8 outdrive support bearing and the end of the case would be handy. I already stuck mine back together but I think the depth is about 0.045".

So I got the part printed. Little disappointed at how the print came out. The part didnt print completely solid, The printer also put supports inside of holes but not all of them. attempted to remove the supports inside the holes would only end up melting parts of the plastic around the hole and would end up ruining it. I did manage to luck out and that only occurred on one hole.

As to my part measurement. I need to check the measurements on my rear shock tower holes, It does fit together with the car but it feels slightly off. The area above the rear arm mounts was only designed to clear the rear arm mounts and the normal screws cant work. I can probably switch to a 1/4" 8-32 screw instead. I should give more clearance around the rear tie rod mounting holes because the top has no clearance for the rod end.

Lastly, I coated the part in some chrome tape for looks.