Building a 450 HP engine in N/A form is going to be much harder and expensive than building one that has the same power with forced induction. A 250 with 450 HP is equivalent to a 350 making 630 HP, so any machine work you would need to perform on the 350 to help it endure that kind of stress, you would also be required to do on the 250. You may even have to dowel the main caps and use main straps if you intend to beat on the engine on a regular basis. Much more extensive headwork will need to be performed to reach this level as well. Also as Hank mentioned, a better induction setup is going to be a must. You also will need to install much more beefier rods, H-Beam or I-Beam style, billet preferably, as well as quality forged pistons. Reaching that HP level in N/A form will require you to go well beyond the compression level needed to run pump gas, also, the peak RPM range on such an engine is going to be well above the level of normal street driving also, and may make street driving altogether a disappointment in a big heavy truck.
On the other hand, using forced induction will give you many benefits over the N/A engine. It will allow you to use pump gas, which you said you would like to do. It also will give you the 450 HP in a much lower RPM range suitable for street driving, and be more pleasant and fun to drive. It will also make your engine last longer because you aren't having to run the engine so hard. You still will need to prep the engine in the same manner as far as machine work and components goes, and maybe add an intercooler and some form of alcohol injection to help fight detonation, but it will definately be the better way to reach your HP goal.
