I feel like a noob but how do you bore out the tbi and set the 18 lb spring ton20 psi lol. Also are you having to run a chip?
As far as boring your throttle body here's where I went. It's fairly expensive but they say it'll flow 650 CFM which is good enough for a 350 @ 6000 RPM. You tell him you want an 18LB spring.
https://www.ebay.com/itm/46mm-PORTE...837306?hash=item3d963196ba:g:mpwAAOSw2xRYWVJj
Then you get a fuel pressure adapter, an AFPR (Adjustable Fuel Pressure Regulator) and a gauge from these guys (around $116)
http://www.rvmorsemachine.com/product.htm
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Here's where I tried out a 28LB spring from a 94-95 7.4L but it wouldn't idle well.
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Now, the fun part - tuning
. My 1st try was a "custom" chip, I wasn't impressed! 2nd shot was soldering in a ZIF (Zero Insertion Force) socket in my 1227747 ECM (88-91) and tuning with Dave w on Gearhead EFI. I'd drive around data logging with an ALDL/USB cable using TunerPro RT, email the log to him, he'd tweak the .bin, email it back to me, and I'd flash an SST 27SF512 flash EEPROM with the Moates Burn2 adapter. Rinse and Repeat until we got the VE tables dialed in.
http://aldlcable.com/aldl.asp
http://www.tunerpro.net/downloadApp.htm
The biggest issue with the 7747 ECM is it's slower than molasses on a winter day, 160 baud rate. Plus the tables only go up to 4800 or less RPM. The EBL Flash-II runs at a 8192 baud rate, its SA and VE tables go up to 6375 and 6000 RPM respectively. It has 8 "banks" you can flash to (each takes about 4 seconds) so, you can tune for different conditions without effecting the others. It comes with a USB cable that plugs into a mini audio socket (I mounted it under the dash for easy access). Plus it has the "VE Learn" function where you drive around and it automatically tunes your VE tables to the AFR you want like 14.7 or a little lower for ethanol level (10% likes 14.1 better). Get yourself a WBO2 and you can even tune WOT levels with this setup.
https://www.dynamicefi.com/EBL_Flash.php
Fun stuff!