I wouldn’t bother with the BP4W from the 99+ miata’s. That engine is setup for RWD, had a short production run so there is limited aftermarket support for it, and most of the tweaks to that engine are mainly beneficial only if you run it N/A.
If you going to do a forged BP build, I wouldn’t bother with the stroker kit. A set of aftermarket 1mm oversized pistons will already get you close to 1.9L and then you can choose whatever compression ratio you want too.
The main problems with the G series trannys is the differential. The pin breaks, comes out the side of the differential, and then takes out the tranny case or the spider gears inside of it strip. The OEM differentials don’t like excessive burn-outs, wheel hop, clutch dumping, or other sudden torque shocks. For the differential you can buy a Mfactory lsd for about $750.
The second issue is the tranny casing itself. When under high power levels(250+FTLb of torque), the case flexs and the gears misalign leading to reduced contact area and stripped gears. Generally 3rd is the most prone to stripping followed by 4th.
As for the case flex/stripping gears, I’ve seen various workarounds. PAR Engeneering makes straight cut gears that eliminate the the secondary shafts side loading the tranny case. A lot of Probe owners just upgrade the third gear and install a lsd, and are doing 300+whp just fine. I know PAR now offers a bearing cup that can be machined/installed into the tranny case too.
There was also a group buy not too long ago on the Probe forums for Mfactory gears. They were offering 500hp semi- helical 3rd and 4th gears as a package deal for $750 or a short ratio set with 3rd, 4th, and 5th for $1000
I’ve even seen somebody that replaced the fifth gear with a bushing and bolted a steel support plate on the end of the tranny in it’s place and ran a KLDE with 20psi of boost. So there are options.
IMHO…I think the E153 is a awesome tranny upgrade for the BG chassis , but I think it would be more expensive and a bigger hassle than what it’s worth trying to get it to fit in a BF chassis. The BG guys are lucky that the Toyota axles and outer CV can be made to work with their 26 spline hubs. I’ve already researched for alternatives for our 24 spline hubs and found nothing, Mazda uses INT type splines for the mid shafts and Toyota uses NO splines, so there’re no hybriding options available for us.
Also going by what Therealdeal posted with his E153 conversion, the larger tranny moved the engine an inch higher and forward too. In order get that to fit our low slopping hood line, a fabricated crossmember will be required to lower the drivetrain and you’ll probably have other clearance issues beyond that too.