4k to 6,200 is a rather large window to be locked up on the big end compared to some combo's racing, imo. Pulling on the big end should not be a problem as long as the converter doesn't have too much slip.
As to the 2nd part, i think you can get there as is, 10.99+, but the cars gonna have to 60'.
Don't know your tire/gear setup, but i ran some calc's with a 3.91/4.10 and 27" that looked about right for the 1/4 mi.
I still personally would work with what you have and get it dialed in. A manifold swap is a afternoons work if you feel it needs it down the road.
