The trouble with a 4bbl conversion is the kickdown linkage. 4bbl vans used to be common in junkyards, not anymore. When I put a 4bbl on my '82 Ramcharger I went to the dealer to source parts, there was not a single kickdown part the same, not even the return springs.

So some backyard fabrication is the cheapest option today?

IIRC, the throttle kickdown linkage to the carb needs to be lengthened an inch or so? In the seventies I once spliced a piece of round rod into one for a conversion, worked fine after adjustment. That's all I recall doing, didn't change or modify anything else.