The Eldo and the Toronado (and the GMC motorhomes) all use the TH425 trans. This is a TH400 bent in half and all the internal parts are common with a TH400, so strength is not an issue. The TH425 has the torque converter and pump behind the engine and the rest of the trans alongside the engine on the driver's side. There's a multi-row roller chain that connects the two at the very back of the trans. All Olds and Caddy motors bolt to this trans, but you need the proper oil pan to clear the RH halfshaft. Yes, all Pontiac and Buick motors bolt up as well, but there are no factory oil pans that have the proper shape. Yes, the Buford V6 was used in longitudinal FWD applications in the 1980s Riviera and Toro, but that was the smaller TH325 trans.
The TH425 differential bolts to the driver's side of the block. There's a shaft the runs under the oil pan to the passenger side with a flange for the RH halfshaft and CV joint. The LH halfshaft and CV joint bolts directly to the differential flange. Stock final drive ratio is 3.07:1 and there is no factory posi unit. The GMC motorhome community has developed both posi and steeper gear sets, but these parts are priced like they are made from solid gold.
Of course the driver's side exhaust manifold is unique to the FWD application since it needs to route up and over the transaxle. There are no Caddy headers available for this configuration. There are Olds headers, again from the GMC motorhome community, and again, they are extremely expensive (apparently all those retirees are pretty well heeled).