bought a mystery 850 double pumper...

Do some measuring. I'd bet that's an early 830. I have one of those main body's for sale here on FABO that I changed to down leg boosters. If no one buys it, I'm taking it to the range and I'm going to blast it to bits.

I'd bet if you do some measuring you'll find it's a 750 base plate with an 850 body. I hate them. I know profession carb guys who won't touch them. I fought mine on and off since 1989.

Buy a billet main body, base and metering blocks and sell the rest. Better yet, sell that and put the money towards a BLP B4X. You'll be happier.