I understand wanting to get it running quicker but you state you don't want to choke it down and stock J heads will choke it down. I highly doubt it'll pull any where close to 6k rpm with them especially since you state you don't want to get into replacing valve springs. Roller cams generally require considerably more spring pressure than flat tappet cams so you will definitely have to replace the valve springs and most likely have the guides cut down and Teflon seals installed because stock J heads wont usually accept over .475 lift without spring retainer to guide contact.
BTW: your last statement about not wanting to run a 3500 converter because you'd have no low end is off base. Converters fall into 2 basic build groups, tight and loose. Nearly all of the pre 1990 high stall converters built were of the loose variety meaning they slipped a lot taking off. Converters have came a long way in the past 20-25 yrs. and today's designs can easily flash 3500 rpm and drive nearly like a stock converter (the tight variety). My Cuda had a 727 with a 3500 PTC converter and a very mild built 408 with a small Racer Brown solid flat tappet cam and I ran a traction limited 12.24 at 112 and to top it off it averages 13-14 mpg around town and 18 on the hwy when I'm not flogging it. If the converter was slipping there's no way it'd ever get anywhere close to that mpg. At idle (750 rpm) when you drop it in gear it takes right off without touching the gas.
