My engine builder is telling me I should get an sfi rated flex plate for my 470 he's building. I have a new & unused but older version of a Mopar Performance flex plate, part number P4529751. However, it doesn't have the sfi sticker on it like the current ones do. Do you think it will be O.K. for my 470 or should I get something else like one of the round style B&M makes? Also, is there any reason to not reuse the factory grade 8 flex plate and torque converter bolts or should I replace them with new?

I use both the SFI and non SFI Mopar flex plates with the stock hemi size(7/16 converter bolts ) OEM bolts all the time, no problems so far after hundreds of Mopar motor builds
If the bolts torque okay use them
EDIT: All the stock OEM Mopar 7/16 or 1/2 inch crankshaft bolts are grade eight, same thing on the converter bolts The OEM grade eight bolts will have markings on the head of the bolts just like after market Hi Po fasteners do

If you plan on upgrading the converter, I would go ahead and update the flex plate and the hardware. You are probably looking at twice as much torque with what you are putting together so converter or not, I would get the good stuff. It's not very expensive and it's nice to have all that stuff new and not worry about it.

So if the new MP flex plate I have isn't good enough for my application, what flex plate would be a good choice?

B&M

I have several fellow racers and friends that bought B&M for their BB and got the short end of the deal, one flex plate bolt hole was off
They had to elongate that hole for the bolt to fit

A quick search shows the B&M as the only option for an external balance 360/727 combo. Are there alternatives?

Just as an FYI, I had a B&M unit for my SS hemi & had a monstrous vibration while making passes. Couldn't understand what was happening. I rebalanced wheels, aligned it & vibration was still there. I took it off & replaced with SFI mopar unit & never looked back. I gave the B&M unit away. Never had a problem with either mopar SFI or non SFI flex plate. Looking at the B&M unit, other than being round, couldn't for the life of me figure why it was so unbalanced, I'm talking major seat of your pants vibration. My $ .02 Dave SS/ea #1355