Problem is if you gonna need that bigger valve your talking big power. When you flycut the piston in the motor your messing with the engines balance. Take your motor out. Put a reemer on each cylinder to get rid of the high spot in the top of the cylinder and pull them with the crank at TDC for the cylinder. Mark everything. Inspect your block crank rods and pistons for cracks. Check your pistons for scuffing on the skirts. Mic your crank and make your its not egg shapped. Also check the rods arent egg shaped. I would put a stock 5.0l past 450flywheel hp above 5500, but there are somethings you can do to make it safer. Balance your entire rotating assembly. Shotpeen your rods and have arp rods bolts installed and most importantly resized. Shit though jegs has eagle forged rods for 265 part # 356-SIR5090FP and they already got arp bolts and i got you the hookup on that stuff if you want it so it would cost less. The 5.0l blocks gonna break before those rods pistons on the crank does.