You're going to need both to get max benefit. I'd do the suspension first and just foot brake the stock converter as best you can until you can do the stall. What converter are you looking at btw? Because a good one is going to be $600-$1200 depending on where you buy from. Freakshow is on the budget end but makes really good converters while PI is going to be at the top and is one of the most trusted/well known stall converter manufacturers out there. 9/10 track cars will have purple PI converters in them.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.