Get a cam closer to 290 advertised duration, and a single plane intake...forget the spacers. Stay under .525" lift unless your going to put some serious springs in the heads.
Might want to do the bottom end before you do this, the stock pistons crack the skirts off and have a habit of throwing the rod through the block from the inbalance at 6500 rpm.
I have a nice collection of 351W rods that I had to dig out of the pavement from back in the day when for $75 you could pick up another engine from the wrecker in good running condition and throw on your parts and do it again!
The 1969-71 rods are good for 7000 rpm, the stock pistons are good for 5500rpm max. She won't bang shifts at 6000 rpm for long before you crack the skirts dude, the oil control ring drain slots on the factory cast pistons are a huge weak point. Factory springs will valve float at 5700 rpm with a stock cam and with a .500" cam...5500 rpm. Likely your tachometer is off, Ford tachs are notoriously inaccurate.
Been there done that.
As for porting...spend your time on the exhaust and intake bowls and get the ports matched. Most of the heads potential is restricted due to these pinch points. Unshroud the exhaust valve on the chamber side and leave the rest alone. Factory heads can have the valve support hump ground off in the intake and exhaust ports if you bronze guide the hole. Doing much more will just be cosmetic. Replace the factory pushrods, they are noodles and flex like crazy at even 5000 rpm.
Get the exhaust right and carb tuned and you should see close to 400Hp with that combo with a peak rpm potential of 6500 rpm.
Remember what I said about the factory pistons...they WILL
crack if you keep abusing them.