I wouldn't use Sierra stuff personally. A gearbox tunnel is easy enough to fabricate and weld in and if you're thinking of using a V8 then you certainly don't want the shit Sierra rear suspension and weak diff - plus it's harder to narrow.

I'd do what I've seen a few well built RWD converted cars have done - strengthen the front end/chassis rails then chassis mount the engine. Fabricate/adapt the bulkhead and add gearbox tunnel and if needed, a prop tunnel. Then I'd fit a narrowed live axle out back on a four-link kit.

Relatively easy to do if you have the skills and relatively cheap too.