I can't say for sure, but I would guess the stock mounting points are probably good up to around to 700hp, maybe more. With a front and rear motorplate you can put the engine wherever you like. The AJE crossmember I have, has 1.5" dropped steering mounts. I lowered the engine down to just about a 1/4" above the rack and pinion and back to where the bellhousing bolts where also a 1/4" from the firewall.
I don't think power oower is the reason people go with motor plates. I believe the biggest reason because of engine placement and the extra room created by the lack of motor mounts. With an engine plate, you can put it where you want/need it.
I don't recall the brand of headers I'm using. I do know that they went out of business and are no longer available.
I bought a complete motor plate kit from D&D. It came with the front and mid plates, brackets, and lateral movement limiters.
The k-member is from PA Racing. I purchased it with a 1.5" dropped Pinto r&p and without motor mounts. I did not lower the k-member with spacers or any other method. Currently, I have the motor about .375" off the r&p. With the motors' placement, the motor will fit under a factory hood.