I'm doing a swap on my USDM 95 Accord LX that had a stock F22B1 engine with a JDM F22 VTEC. It's supposed to be the same as the USDM EX model of that generation using the SOHC VTEC engine.

I'm no mechanic but did much research that "For the F22B SOHC VTEC vs. non-VTEC exhaust manifolds, there is no major visual difference in the exhaust port shape or bolt pattern at the cylinder head between the two versions. Both VTEC and non-VTEC F22B SOHC engines share the same head casting and exhaust port layout at the manifold flange. The main difference is in the manifold collector and downpipe flange design (4-2-1 for VTEC, 4-1 for non-VTEC), not the head flange."

BUT for a "Non-VTEC manifold on VTEC head, The smaller D-shaped ports of the non-VTEC manifold will restrict airflow to the VTEC engine’s larger oval ports, reducing performance." Supposedly this is the difference:
Non-VTEC D-shaped (~35mm × 25mm) vs VTEC Oval (~38mm × 30mm).

I then bought headers online which I had the ports measured as 41.5mm (width) x 34mm (height), slightly larger than the commonly cited ~38mm x 30mm for F22B SOHC VTEC (F22B1) exhaust ports, but this is not unusual or problematic.I understand that to fit a 4-2-1 header on my JDM F22B SOHC VTEC engine in a USDM 1995 Accord LX, I will likely need to modify or replace the downpipe to match the new header’s collector flange and length.

My mechanic, who has no experience with swaps, says that the new headers don't fit even though I verified w/ the seller that they fit the EX model. Furthermore, he bolted on the original LX exhaust manifold with supposedly smaller ports to the engine and says it fits fine.

I'm wondering if I should insist that he revisit not using the original LX exhaust manifold since I've read for a Non-VTEC manifold on VTEC head, The smaller D-shaped ports of the non-VTEC manifold will restrict airflow to the VTEC engine’s larger oval ports, reducing performance. Are there any more serious issues or long term consequences leaving it as it is?

Any advice is greatly appreciated.​​​​​

 

There are a few differences between the B1 and B2.

B2 have the HO2S in the manifold
B1 have the HO2S in the down pipe under the steering rack.

Not only is there a flow difference in the 4-2-1 B1 exhaust vs the B2, but the location of the HO2S also affects the reaction time in closed loop.
ECU is programmed for t time from the HO2S, while in closed loop the ECU will monitor HO2S readings, since the sensor would not move the reaction time for the ECU is a known factor at a given rpm. It will know that at a given rpm when the sensor tells the ECU a cylinder is rich/lean the ECU will know which cylinder it was and correct for that cylinder. If the HO2S is in the wrong physical position, the timing from when the exhaust reaches the HO2S will be incorrect and the ECU will try to correct for the wrong cylinder.
Installing the HO2S in the wrong spot will cause for advanced or retarded signaling, this will affect how the ECU reacts which will throw off the tune and running condition of the engine in closed loop.