$1200, SBC, 450/450 HP/TQ, Tuned Port is not a feasible combination.
Routes you could take.
Vortec heads, tpi Vortec intake.
$200-300 for good stock heads
$350-400 for tpi base intake for Vortec heads
lift is limited to 0.550" or so without cutting valve guides, beehive ls6 spring swap is about $100-125 for gmpp yellow or blue springs and retainers/locks for 11/32 valves. If Speed Density TPI, add in a tune. If MAF TPI, be careful with your cam selection to keep the ecm from flaking out.
Option 2
Import aluminum heads (ProComp etc), shoot for 200cc or so intake runners. Mod an LT1 intake to fit with a remote thermostat setup. There are still guys that will bore the lt1 intake to accept a standard distributor. Can and springs. Get it tuned. The larger runners will get you more flow without going too big on cam and making the ecu go crazy. Stock 48mm lt1 throttle body flows pretty well, though internet lore will tell you otherwise. Still, it's a 400/400 kind of setup.
Heads -$800-900 new, cam/springs can be done for $300 if you buy used. Keep in mind I'm assuming this is a late 80's+roller cam core motor.
Option 3 LSX.. carbed is cheapish, getting to 450/450 is an iron 6.0 short block, 243/799 heads, and a relatively mild cam and springs.
The devil is in the details, headers, accessory drive. Wiring has costs if efi is a necessity. You can do a budget Ls swap, but there are some costs that are hard to get around.
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