My matco 3/8" locking flex is probably the most beautiful tool ever to grace my box. But yeah.
Full set of sockets, standard deep and sometimes others for example my head bolts are 12 point bolt head 12MM. 13mm fits perfect but if you're dumb enough to use it, you've just ruined the bolt.
Solid and swivel extensions in lengths up to 6"
Feeler gauges, plastigauge
Rubber mallet, vice grips with some good rags for cam/crank end caps
Torque wrench...Go through your service manual and find out the maximum torque specs your motor will require and buy a wrench accordingly.

If you're building a full out race motor. Don't rebuild it yourself. Engine builders are good at what they do because they check ever clearance possible and know whats acceptable and whats not. For a good street motor, you don't need to go that crazy. Have a machine shop bore and hone, use the correct oversize bearings. Make sure your bearing clearances good. Throw everything together to the right torque specs and party.