Why would anyone want to replace an RB30 with an Ecotech which is much older technology ???
RB30's are killer engines and far superior to any Buik based product.
N/A for N/A, EFI and head design on the Ecotec is way better than the single cam jam, not to mention more displacement/power/torque and probably more fuel efficient. Ecotec is purely CAS based and doesn't have a dizzy either, also got a much more tunable factory ECU. Sure, it's got push rods but an RB30 is only a 2 valve motor as well and they probably weigh about the same. Only thing the 30 has got going for it is ET with lower compression, in which case, yes, the RB30 is a much better base for power. It's way more obvious with a 5 speed behind them instead of a slush box.
Big bonus though is that you can buy an Ecotec plentifully/cheaply from 2004ish vs. an RB30 from 1988.
VN-VS K-frame will bolt into the VL. Use the VL knuckles and uprights as the VN-VS ones are likely to have different ackerman. Will probably have to make up a transmission mount. If the tailshaft doesn't fit try swapping the front and rear halves between the two. Easiest way to get around security is to get a mail order memcal with VATS turned off, will probably cost you a hundred or so but is worth it. The1 and Delcowizzid on this forum can do it for you, or you can got to the PCMHacking forums.