400 horse from a street 340 is relatively easy, it involves a good intake, reasonable sized cam, headers and exhaust.
Eddy Rpm air gap with 750 cfm carb annular booster preferable air cleaner with a 14x4 air filter for zero restriction
cam shaft with approximately .5 inch and 265-275 intake duration
1 5/8 headers with 2.5 inch exhaust H-pipe / X-pipe is mandatory
9.5-1 compression 2.02/1.60 valves w/ 3 angle valve job
This should get between 365-390 horse, add some cylinder head porting , some intake porting and gasket matching, 400 is easy mode, with enough vacuum to run power brakes. However, if you are trying to maintain a totally factory appearance 350 is a more realistic number. Than your talking extrude honing the intake and exhaust manifolds, head work, etc.
The price that guy quoted you is stupid high, I just help a friend do a 392 striker w/rhs heads and efi for less lol.