Looking for the right path!

I would back off of the mid 400 HP range as said for a budget. If you are new to muscle cars, you will have your hands full driving a true 325-350 HP anyway; it is not like a peaky 350 HP in rice burner; you will have gobs of torque in the low and mid range.

The idea of 'streetable' begs some more questions; a LOT of pretty wild engines are 'streetable'. Will you use this as a daily driver? Does fuel mileage matter? This all works into cam and piston selections.

Again, can you give any ideas on the budget limits?

IIRC, those 73 heads have 1.88 valves. If you can afford it, moving up to 2.02" valves would help the top end HP. That is the limit for reaching anywhere close to 400 HP BTW; the breathing is not there. If you have the money for some tools, a place to work, the patience and time, you could do some head porting yourself to save money. But it takes a lot of careful work and time, and some folks just should not do that.

The 73 is a lower compression ratio engine and if this is for roaring around the backroads, then it would be wise to get the CR back up to a true 9 to 9.5 range. This will help a lot with low RPM torque, which you will use coming hard off of corners as you romp around the back roads.

As for suspension: (Yeah, I know... this is the engine section!) With the roads in your neck of the woods, they are not all that curvy (I live out in the Blue Ridge region of VA) but they can be bumpy and rough in spots. So don't go with the stiffest springs and front torsion bars; those are suitable for flat roads and auto-x, but not for keeping as much suspension travel as you can. You REALLY need to save your pennies for a set of Bilsteins; the result will be nothing short of spectacular on those roads. And be looking at anti-sway bars of course. Poly bushing will tend to beat up the suspension parts on rough roads, so I would stick with rubber when renewing the suspension parts.