putting in a new engine need advice on trans

[lugnut76]

OK hears the deal my old 396/402 is toast. So I am having a local engine shop build a mild 454/425HP, good street drivable engine. rear gear 12 bolt rear end with a Moser posi and 373 gears.

(I did not install this it came on the car)
I have a 350 Turbo Trans with a 2500 Converter and shift kit. And I don't like how it shifts. From a dead stop just driving down the road it running up to 23-2500 then shifts HARD, not fun, so I have been manual shifts to get around this.

I am wanting to know what converter could I install or I also have a brand new 350 Turbo with no shift kit in the shop that I could use. Could I just swap the 2500 converter to that?

I don't have the extra funds to go a 400 or 700r route yet.

Thanks for any info/help