Hello,
The '87 5.0 HO came with excellent forged piston from the factory easily capable of handling your horsepower requirements.
Depending on the milage and condition of the short block, you may be able to use your original pistons with a fresh hone and new rings.
If not DSS
www.dssracing.com offers excellent replacement pistons that will fit most any aftermarket head/camshaft.
I would never bore a late model 5.0 more than .030"
Stock cylinder heads (E7TZ) have a 64 cc combustion chamber.
The stock EEC IV ignition system is more than capable of handleing 350 hp
I believe the stock spark plug is a Motorcraft AWSF-42C If you change to aftermarket heads, which will be required for your horsepower goals the type of plug may change from a tapered seat to a gasketed seat. The gasket seat plug to use with AFR heads (which I highly recomend) is an AGSP-32C
HTH,
Mike