[Toivo]

You should look at that 390 more closely.

It's a common misconception that the FE series of engines weighs more than a 351. It all depends on how the engines being compared are configured. Because a large part of an FE's topside is manifold, switching from an iron to an aluminum manifold can shave a lot of weight off of the engine. With aluminum heads, manifold, waterpump and headers instead of cast iron logs, an FE can weigh the same or even less than a 351 - again depending on what the 351 has by way of heads, etc.

I don't think you want to base your decision solely on weights, they are actually fairly close. What you do want to consider is whether you can work on this engine yourself or need the services of a shop. Generally (again) speaking, you'll have a greater selection of shops that can work on the 351 family of engines than can work on FE's. Going forward, modifications and parts are probably going to be cheaper for the smallblock as well.

JMO