Thanks for all the replies.
I'll outline the dilemma. I want to go faster, I thought I could do it with more cubes and the block and top end I'm using now. Plan was to buy another set of -13 heads (since then I can use all the other stuff I have) and get them cnc ported. But I was going to buy another set of heads anyway, Predators are out of the question I think they'll work out too expensive for me, and I think the same for the B1 TS.
Not sure about PSO's though, I've no experience.
Should I keep developing the 572 or go to 604.
Seems like the heads I have are holding me back

Also can anyone tell me if headers that fit -13 will fit B1 MC's? (or PSO's)?

I don't want a Hi RPM race gas motor.
We've gone 6.1 @ 114 1/8th 9.57 @ 142 1/4 @ the weight in sig. I shift @ 6500 and motor goes to 6800 at the stripe.
4 figure HP would be nice since we think we're around 850hp given the times, MPH and weight (Cars been weighed on 4 scales now all within 40lb). Thanks again.

Cheers.
Tig

'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials.
9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge.
RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03