Yes, this is most definitely MB blasphemy of the highest order. Yes, parts are a dime a dozen for an SBC. Just goes to show that you get what you pay for.
If you are bent on swapping in an American V8, however, there are several comments I will offer. To begin with, you mentioned that your transmission is in good condition. If you are thinking of mating it with a non MB engine, forget it! You will be much better off using a transmission from the manufacturer that makes the engine.
Secondly, if you want an American V8, the SBC is one of the worst choices for the 126 or most any other MB chassis. This is because the oil pan sump is on the REAR of an SBC, while the MB engine has a front sump. What goes along with this is the fact that the distributor is in the rear too and will be a real problem to fit against the firewall. A small block Ford would be a better choice due to its front distributor and front sump.
If you plan on doing ALL this work yourself including rebuilding and preparing your own engine, fabricating mounts, doing all the jillions of wiring and plumbing changes, and you really watch your budget you MIGHT be able to accomplish this for less than it would take to properly rebuild your MB V8. If you must have this work done, you are entering into a project based on false economy.
In any event, your end result will be a noisy, relatively rough running engine.