There's allot to consider,
IMHFO kind of depend on the blower & style of rotors, who set it up, how new etc.
{like hi-helix rotors with Teflon strips in a 6:71, will or could out perform a 8:71 with std rotors, at the same drive ratios}
All blowers aren't created equal, it really depends on what blower, what case, what material the case is even, what exit opening can make a big difference even {but most all 6:71's & 8:71's are std wide open exits/bottoms} & how it's been set up, whether the case is still good, not all scratched up or worn, what or if it has the proper clearances or the drive is set-up & timed properly, is the still drive good {does it even come with a drive}, it's not just about size...
You need to weight the cost & effects,
what kind of cooling system will you be running ?, {it better be a good one}
what type of fuel are you planning on running ?, {methanol or even higher levels of ethanol, will be allot cooler, will actually cool the blower, allot more maintenance thou too}
what size & type of fuel system will you be running ?, {carbs or FI/mech. or EFI}
what's your intended usage & goal/expectations ?,
what kind of ignition will you be running ?,
Detonation will kill a blower motor bottom end, really fast, it's pretty a must, to be able to back out timing, with any type/amount of boost, even if it's at like a 6psi level, that should really be mandatory too...
what kind of head sealing are you planning ?
what kind of head gaskets ?{that really could be your biggest hurdle/limiting factor}
will it be o-ringed on the deck & receiver groves in the heads ?,
IMHFO if not I'd stick with a smaller blower & under-drive,
how much overdrive or under-drive you want to run ?,
how much boost can your bottom end & top end/head seal handle ?,
what type pistons/compression, rods, bearings, crankshaft, camshaft will your engine be ?
especially what kind of bottom end will you have ?
will you have stock style main caps & bolts ?
IMHFO if so stick with a smaller blower & don't push it too hard,
Mainly because the main webs in a 440 RB block are somewhat weak
{383-400 low deck is stronger smaller line bore main bearings},
BUT you could install billet caps & studs, with a block girdle, then have it line honed & be allot safer, "I'd highly recommend it"
IMHFO the bigger the blower, the more boost, the better head seal & bullet proof bottom end, it's pretty much considered mandatory, when dealing with any kind of boost, especially over 6psi & that's really pushing it even...
How much boost you want, how much power are you trying to make ??
I'd also highly suggest running a good pop off valve {spring operated will work fine for the street} that will release pressure & save the bower & bottom end, when you let off the throttle under high boost, the boost has to have someplace to go/escape...
If it's for racing not just a street car application, then usually I'd say the bigger the better usually applies...
If it's just for the street application, either will probably work just fine, 6:71's have been around for eons, you can double your power output with a 6:71 easily...
The 8:71 will make more boost at a lower RPM, with the same over or under-drive, if they were set up equally...
Either one will produce more heat, the tighter the blower {better boost} the more heat it will generally create...
I almost forgot the camshaft,
is it ground for a blower or boosted application ?? {could/will be a limiting factor too}
I'd highly suggest you talk to a cam guru & get the proper camshaft for you specific needs & your specific combo...
Sorry for the long winded overly **** retentive post, or barrage of **** questions,
that you'll need to address yourself....
But there's many other factors to consider, other than just size or cost,
especially if you want it to live, for any period of time, it's a big expense either way & it absolutely needs to be set-up properly, with the proper components, for what you ultimately end up with...
Good luck & have fun,
there's nothing like seeing & hearing a big ol'huffer on a BB Mopar,
it give me "goose bumps" just thinking about it...
