I'm about to put a 250 on a stock 454 mag. Any pointers on jetting carb? The top plate is setup for a single 4150 Holley. Seems like not enough to me. Can a 4150 be jetted to make it close to 1000cfm? Also should I change head gaskets? And to what kind?

Sell me one of those splined pulleys and ill tell ya ..
jk tbe b&m guy says the single whether centered or offset to the rear is doomed . Lots of guys running them though. I plan on using a 1050 dominator and tuning with the "house " dry headers . Unless someone sells me the dual top plate i really need really cheap.will post all results once i get the upper pulley situation fixed

Jetting changes does not change the cfm.

I would look for a 4500 carb plate go with a 1050cfm Dominator minimum.

yeap, jetting not going to change CFM, try calling patrick @ prosystems, get his input, he did a blower carb for a 177 blower for me and was spot on....