I made this thread as a photo-journal for my upcoming build, to give ideas to other folks who may be interested in something like this and to ask for help for the specific project all in one place (it’s likely that I’ll stumble upon many problems and I don’t want to flood the forum with threads everywhere for each one 
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Some story:
Back in 2007 I had built a not-so-cool MBSID with two 6581s (I never got it to work in stereo, still don’t know why, tried everything) that was pretty ugly, had a Minimal CS and was fiddly at best. Had some problems with the C64 PSU and the “optimized circuit” (latching transistors sometimes hang up if caps not fully empty, so the +5V rail failed upon startup), stereo wasn’t working, one of the 6581 had a messed up filter, so I ended using it as mono, anyway. I was always frustrated with the terrible SNR performance of the 6581, so when I use it in the studio I always noise-gate it, and when I take it with me live (I play at various bands - mainly synthbass parts) I always ask for the sound engineer to noise-gate it (although same couldn’t happen on stage because I usually feed it directly to a bass-amp for monitoring). I ended up not using it regularly (as much as I love its sound) not only because of the beforementioned problems with the build, but also because I didn’t want to carry so much stuff with me (“proper” synth, MBSID, extra master keyboard- I don’t like layering and splitting, I want to have every single note available to one instrument at a time).
So, the other day, a client/friend of mine gave to me an Evolution MK-361c, which is a master keyboard (now discontinued, company bought by M-Audio - I think). “I don’t ever use it, I’m a singer, your a keyboard player, so why don’t you have some fun with it?”, he said. The problem was I already have a (better) master keyboard, so didn’t know what to do with it. I am an electronic device hoarder, though, so I thought i’d keep it. 
Then it occured to me: what if I merge the Evolution MK and the MBSID into an all-powerful standalone synth? It would have its own keyboard, and I have plenty of case real-estate to fit some other stuff in there, such as:
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an analog filter (as good as the 8580 filter is in comparison with 6581, it will never match a discreet analog filter imho). I chose the Steiner VCF, which is a diode-ladder design, has plenty of character, is multimode, sounds good enough, is easy to build and has CV control.
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a simple DIY digital delay with modulation (this one in particular: http://musicpcb.com/pcbs/echo-base-delay/), which I have already made in a stompbox and rarely use it, can’t get the LFO behave properly, but I’m hoping to fix it. I am a big fan of “analog-type” filtered delays in pads and leads, as well as some slap delay for fast-decaying sounds - I always thought analog synths sound “dry” on their own.
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a big muff (fuzz) clone for that dirty synth bass (I play a lot of synthbass).
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an small 2ch active mixer to control the mix of the two SIDs. I’ll have one 6581 and one 8580 so I can blend them together and even route each one to a different input of the filter via a switch (LP,BP,HP). This way I can keep the moody/distorted/filtered low end of the 6581 and the sparkly top end or midrange of the 8580 and do all kinds of crazy combinations.
Well these are the key features that make it a new twist on the average project. Apart from that it will feature a lot of default stuff:
-Knob #1 to #5 as analog pots, directly on core module
-Minimal Control Surface
-7 internal banksticks #1-#7, a 9DIN input for BANK #0 (I already have a couple of bankstick “cartridges”)
-AOUT_LC module for controlling the Steiner VCF
-Meanwell RPT-60B switching PSU, because I need a -12V rail as well (some modules use inverting opamps). It’s small, very likely to be in the synth casing and not in the form of an external brick)
-Master Volume control
-Pitch Bend and Mod Wheel (kept from the Master Keyboard)
-Octave Up/Down buttons (kept from the Master Keyboard)
-3PDT switch to route the MIDI IN of the Core Module to either the MIDI OUT of the Master Keyboard Contoller for live playing or the back panel MIDI IN jack for sequenced playback and firmware upgrades etc. (no money or space -and probably use- for a MIDI merger)
The only problem is that the Master Keyboard is 5 octaves long, and I don’t need that much, I want a compact synth!
But when I opened the case I knew it would be easy to cut it down to 3 octaves. SO I DID! (the pcb was cut using a dremel, the keys are interchangable in groups of 1 octave each and the main mounting plate was cut using an electrical saw). AND IT WORKS 
I ordered the wood from a local store, yesterday I build the main casing which is the bottom, side panels, a post to support the keyboard and a front post that fills the gap from the bottom up to the lowest point of the key when it is pressed:
VOILA:
I’m very happy with it for now, all goes well!
If I’m forgetting something obvious that will become a show-stopper later and you find it, please do tell!!!
Its difficult to predict everything on my own when I’m disigning such a complex (for me) instrument!