1) Get the EBL from Dynamic EFI. Bob knows his stuff and will help you tune it yourself. Nobody cares about your tune as much as you, that should be insentive enough to not let anyone else touch your stuff. the EBL is designed for use with TBI and can be converted to MPFI if you choose to go that route. Also has lean cruise features and is able to control a 2 or 3 bar Map sensor.
2) TBI - You need to know what setup your whipple is made for. If it's a 350 whipple (probably is) then you can use the 454 TBI but you'll have to bore out the plate on the whipple (TBI side & Intake Side) and the intake, or go to a different intake if they have one availiable. Look into the later TBI intakes, I dunno about the 4.3 but the 7.4 intakes changed to a better design in the later years (92+ I think).
3) Injectors, you probably will want to talk to bob about the injectors, I have no clue what the fuel demands on a blown 4.3 are.
Alot of people are gonna tell you not to put this much work into your 4.3. I say go for it, it's different and will probably make a little bit of power. While it's running you can build a 383 and then just bolt your whipple on it, make some tune changes and be good to go. Just my .02 btw.
I have whipplecharged, cammed, intake, head, ect, ect, ect 454 SS with EBL. It's all waiting to be installed right now (currently deployed) but after months of research this was the best option for me.