Nice car, any pictures?
I had a '68 4-door hardtop back in the 1980's.
The car should have a 8-3/4" rear end and 727 transmission. Both can take alot of power. If in good shape, with a shift kit, can handle around 500 HP and even more with a few modifications.
You can stroke the 383 to around 496" (actual size depends on bore oversize.) That is a 4.25" stroke crank.
The 440 source stroker kits are good for the price.
I haven't priced any of the fancier cranks, rods, pistons in 10 years, so not sure how the prices stack up?
There are a bunch of aftermarket heads to choose from. For something not to crazy (Hydraulic Flat tappet cam?) I would look at heads that accept stock style rocker arms.

On the Milider and maybe easier side of things, there is a guy in Centennial, CO. who is selling a freshly rebuilt 440. I don't know anything about it, but it might be worth checking out?